diff --git a/CHANGELOG.md b/CHANGELOG.md
index faf4bd54280c..06d9630803bf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ Bug fixes:
 * Fix `IO#{wait,wait_readable,wait_writable}` methods and switch the current thread into a sleep state (@andrykonchin).
 * Fix `rb_global_variable()` for `Float` and bignum values during the `Init_` function (#3478, @eregon).
 * Fix parsing literal floats when the locale does not use `.` for the decimal separator (e.g. `LANG=fr_FR.UTF-8`) (#3512, @eregon).
+* Fix `IO#{read_nonblock,readpartial,sysread}`, `BasicSocket#{recv,recv_nonblock}`, `{Socket,UDPSocket}#recvfrom_nonblock`, `UnixSocket#recvfrom` and preserve a provided buffer's encoding (#3506, @andrykonchyn).
 
 Compatibility:
 * Move `IO#wait_readable`, `IO#wait_writable`, `IO#wait_priority` and `IO#wait` into core library (@andrykonchin).
diff --git a/lib/truffle/socket/basic_socket.rb b/lib/truffle/socket/basic_socket.rb
index 7bfee710d74f..aa58725fdb8a 100644
--- a/lib/truffle/socket/basic_socket.rb
+++ b/lib/truffle/socket/basic_socket.rb
@@ -163,7 +163,11 @@ def send(message, flags, dest_sockaddr = nil)
       end
 
       str = buf.read_string(n_bytes)
-      buffer ? buffer.replace(str) : str
+      if buffer
+        buffer.replace str.force_encoding(buffer.encoding)
+      else
+        str
+      end
     end
   end
 
diff --git a/lib/truffle/socket/ip_socket.rb b/lib/truffle/socket/ip_socket.rb
index 2a75a71a10cd..72e11c4c6183 100644
--- a/lib/truffle/socket/ip_socket.rb
+++ b/lib/truffle/socket/ip_socket.rb
@@ -61,7 +61,9 @@ def peeraddr(reverse_lookup = nil)
       end
     end
 
-    message = buffer.replace(message) if buffer
+    if buffer
+      message = buffer.replace message.force_encoding(buffer.encoding)
+    end
 
     [message, [aname, addr.ip_port, hostname, addr.ip_address]]
   end
diff --git a/lib/truffle/socket/socket.rb b/lib/truffle/socket/socket.rb
index 1dbe31226359..188c0806ebd7 100644
--- a/lib/truffle/socket/socket.rb
+++ b/lib/truffle/socket/socket.rb
@@ -384,7 +384,9 @@ def recvfrom(bytes, flags = 0)
     if message == :wait_readable
       message
     else
-      message = buffer.replace(message) if buffer
+      if buffer
+        message = buffer.replace message.force_encoding(buffer.encoding)
+      end
       [message, addr]
     end
   end
diff --git a/lib/truffle/socket/unix_socket.rb b/lib/truffle/socket/unix_socket.rb
index 7b8a5bdf53e4..491b29862e42 100644
--- a/lib/truffle/socket/unix_socket.rb
+++ b/lib/truffle/socket/unix_socket.rb
@@ -58,11 +58,18 @@ def initialize(path)
     Errno.handle('connect(2)') if status < 0
   end
 
-  def recvfrom(bytes_read, flags = 0)
+  def recvfrom(bytes_read, flags = 0, buffer = nil)
     Truffle::Socket::Foreign.memory_pointer(bytes_read) do |buf|
       n_bytes = Truffle::Socket::Foreign.recvfrom(Primitive.io_fd(self), buf, bytes_read, flags, nil, nil)
       Errno.handle('recvfrom(2)') if n_bytes == -1
-      return [buf.read_string(n_bytes), ['AF_UNIX', '']]
+
+      message = buf.read_string(n_bytes)
+
+      if buffer
+        message = buffer.replace message.force_encoding(buffer.encoding)
+      end
+
+      return [message, ['AF_UNIX', '']]
     end
   end
 
diff --git a/lib/truffle/stringio.rb b/lib/truffle/stringio.rb
index 5eb4d0918e65..702e02a1e5cf 100644
--- a/lib/truffle/stringio.rb
+++ b/lib/truffle/stringio.rb
@@ -450,6 +450,8 @@ def read(length = nil, buffer = nil)
       pos = d.pos
       string = d.string
 
+      # intentionally don't preserve buffer's encoding
+      # see https://bugs.ruby-lang.org/issues/20418
       if length
         length = Truffle::Type.coerce_to length, Integer, :to_int
         raise ArgumentError if length < 0
diff --git a/spec/ruby/core/io/pread_spec.rb b/spec/ruby/core/io/pread_spec.rb
index 28afc80e5c61..6d93b432c263 100644
--- a/spec/ruby/core/io/pread_spec.rb
+++ b/spec/ruby/core/io/pread_spec.rb
@@ -22,7 +22,7 @@
 
     it "accepts a length, an offset, and an output buffer" do
       buffer = +"foo"
-      @file.pread(3, 4, buffer)
+      @file.pread(3, 4, buffer).should.equal?(buffer)
       buffer.should == "567"
     end
 
@@ -38,6 +38,13 @@
       buffer.should == "12345"
     end
 
+    it "preserves the encoding of the given buffer" do
+      buffer = ''.encode(Encoding::ISO_8859_1)
+      @file.pread(10, 0, buffer)
+
+      buffer.encoding.should == Encoding::ISO_8859_1
+    end
+
     it "does not advance the file pointer" do
       @file.pread(4, 0).should == "1234"
       @file.read.should == "1234567890"
diff --git a/spec/ruby/core/io/read_spec.rb b/spec/ruby/core/io/read_spec.rb
index eb3652e692a5..8741d9f0179a 100644
--- a/spec/ruby/core/io/read_spec.rb
+++ b/spec/ruby/core/io/read_spec.rb
@@ -376,6 +376,21 @@
     buf.should == @contents[0..4]
   end
 
+  it "preserves the encoding of the given buffer" do
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    @io.read(10, buffer)
+
+    buffer.encoding.should == Encoding::ISO_8859_1
+  end
+
+  # https://bugs.ruby-lang.org/issues/20416
+  it "does not preserve the encoding of the given buffer when max length is not provided" do
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    @io.read(nil, buffer)
+
+    buffer.encoding.should_not == Encoding::ISO_8859_1
+  end
+
   it "returns the given buffer" do
     buf = +""
 
diff --git a/spec/ruby/core/io/readpartial_spec.rb b/spec/ruby/core/io/readpartial_spec.rb
index 0060beb54504..0852f20b2da8 100644
--- a/spec/ruby/core/io/readpartial_spec.rb
+++ b/spec/ruby/core/io/readpartial_spec.rb
@@ -62,7 +62,7 @@
     buffer = +"existing content"
     @wr.write("hello world")
     @wr.close
-    @rd.readpartial(11, buffer)
+    @rd.readpartial(11, buffer).should.equal?(buffer)
     buffer.should == "hello world"
   end
 
@@ -106,6 +106,7 @@
     @wr.write("abc")
     @wr.close
     @rd.readpartial(10, buffer)
+
     buffer.encoding.should == Encoding::ISO_8859_1
   end
 end
diff --git a/spec/ruby/core/io/sysread_spec.rb b/spec/ruby/core/io/sysread_spec.rb
index 003bb9eb9491..8851214283ca 100644
--- a/spec/ruby/core/io/sysread_spec.rb
+++ b/spec/ruby/core/io/sysread_spec.rb
@@ -97,7 +97,7 @@
 
   it "discards the existing buffer content upon successful read" do
     buffer = +"existing content"
-    @file.sysread(11, buffer)
+    @file.sysread(11, buffer).should.equal?(buffer)
     buffer.should == "01234567890"
   end
 
@@ -107,6 +107,13 @@
     -> { @file.sysread(1, buffer) }.should raise_error(EOFError)
     buffer.should be_empty
   end
+
+  it "preserves the encoding of the given buffer" do
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    string = @file.sysread(10, buffer)
+
+    buffer.encoding.should == Encoding::ISO_8859_1
+  end
 end
 
 describe "IO#sysread" do
diff --git a/spec/ruby/library/socket/basicsocket/recv_nonblock_spec.rb b/spec/ruby/library/socket/basicsocket/recv_nonblock_spec.rb
index 17c846054df1..df42c116fb33 100644
--- a/spec/ruby/library/socket/basicsocket/recv_nonblock_spec.rb
+++ b/spec/ruby/library/socket/basicsocket/recv_nonblock_spec.rb
@@ -52,9 +52,19 @@
       @s2.send("data", 0, @s1.getsockname)
       IO.select([@s1], nil, nil, 2)
 
-      buf = +"foo"
-      @s1.recv_nonblock(5, 0, buf)
-      buf.should == "data"
+      buffer = +"foo"
+      @s1.recv_nonblock(5, 0, buffer).should.equal?(buffer)
+      buffer.should == "data"
+    end
+
+    it "preserves the encoding of the given buffer" do
+      @s1.bind(Socket.pack_sockaddr_in(0, ip_address))
+      @s2.send("data", 0, @s1.getsockname)
+      IO.select([@s1], nil, nil, 2)
+
+      buffer = ''.encode(Encoding::ISO_8859_1)
+      @s1.recv_nonblock(5, 0, buffer)
+      buffer.encoding.should == Encoding::ISO_8859_1
     end
 
     it "does not block if there's no data available" do
diff --git a/spec/ruby/library/socket/basicsocket/recv_spec.rb b/spec/ruby/library/socket/basicsocket/recv_spec.rb
index 9fe8c52f9a52..e82a357d3dfd 100644
--- a/spec/ruby/library/socket/basicsocket/recv_spec.rb
+++ b/spec/ruby/library/socket/basicsocket/recv_spec.rb
@@ -100,13 +100,29 @@
     socket.write("data")
 
     client = @server.accept
-    buf = +"foo"
+    buffer = +"foo"
     begin
-      client.recv(4, 0, buf)
+      client.recv(4, 0, buffer).should.equal?(buffer)
     ensure
       client.close
     end
-    buf.should == "data"
+    buffer.should == "data"
+
+    socket.close
+  end
+
+  it "preserves the encoding of the given buffer" do
+    socket = TCPSocket.new('127.0.0.1', @port)
+    socket.write("data")
+
+    client = @server.accept
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    begin
+      client.recv(4, 0, buffer)
+    ensure
+      client.close
+    end
+    buffer.encoding.should == Encoding::ISO_8859_1
 
     socket.close
   end
diff --git a/spec/ruby/library/socket/socket/recvfrom_nonblock_spec.rb b/spec/ruby/library/socket/socket/recvfrom_nonblock_spec.rb
index 94f58ac49f4a..5596f91bb86b 100644
--- a/spec/ruby/library/socket/socket/recvfrom_nonblock_spec.rb
+++ b/spec/ruby/library/socket/socket/recvfrom_nonblock_spec.rb
@@ -52,6 +52,27 @@
           end
         end
 
+        it "allows an output buffer as third argument" do
+          @client.write('hello')
+
+          IO.select([@server])
+          buffer = +''
+          message, = @server.recvfrom_nonblock(5, 0, buffer)
+
+          message.should.equal?(buffer)
+          buffer.should == 'hello'
+        end
+
+        it "preserves the encoding of the given buffer" do
+          @client.write('hello')
+
+          IO.select([@server])
+          buffer = ''.encode(Encoding::ISO_8859_1)
+          @server.recvfrom_nonblock(5, 0, buffer)
+
+          buffer.encoding.should == Encoding::ISO_8859_1
+        end
+
         describe 'the returned data' do
           it 'is the same as the sent data' do
             5.times do
diff --git a/spec/ruby/library/socket/udpsocket/recvfrom_nonblock_spec.rb b/spec/ruby/library/socket/udpsocket/recvfrom_nonblock_spec.rb
index 650a061221b8..b8040995897c 100644
--- a/spec/ruby/library/socket/udpsocket/recvfrom_nonblock_spec.rb
+++ b/spec/ruby/library/socket/udpsocket/recvfrom_nonblock_spec.rb
@@ -58,6 +58,15 @@
             buffer.should == 'h'
           end
 
+          it "preserves the encoding of the given buffer" do
+            buffer = ''.encode(Encoding::ISO_8859_1)
+            IO.select([@server])
+            message, = @server.recvfrom_nonblock(1, 0, buffer)
+
+            message.should.equal?(buffer)
+            buffer.encoding.should == Encoding::ISO_8859_1
+          end
+
           describe 'the returned Array' do
             before do
               IO.select([@server])
diff --git a/spec/ruby/library/socket/unixsocket/recvfrom_spec.rb b/spec/ruby/library/socket/unixsocket/recvfrom_spec.rb
index fedf74bb2fa2..d849fdc30264 100644
--- a/spec/ruby/library/socket/unixsocket/recvfrom_spec.rb
+++ b/spec/ruby/library/socket/unixsocket/recvfrom_spec.rb
@@ -31,6 +31,29 @@
       sock.close
     end
 
+    it "allows an output buffer as third argument" do
+      buffer = +''
+
+      @client.send("foobar", 0)
+      sock = @server.accept
+      message, = sock.recvfrom(6, 0, buffer)
+      sock.close
+
+      message.should.equal?(buffer)
+      buffer.should == "foobar"
+    end
+
+    it "preserves the encoding of the given buffer" do
+      buffer = ''.encode(Encoding::ISO_8859_1)
+
+      @client.send("foobar", 0)
+      sock = @server.accept
+      sock.recvfrom(6, 0, buffer)
+      sock.close
+
+      buffer.encoding.should == Encoding::ISO_8859_1
+    end
+
     it "uses different message options" do
       @client.send("foobar", Socket::MSG_PEEK)
       sock = @server.accept
diff --git a/spec/ruby/library/stringio/shared/read.rb b/spec/ruby/library/stringio/shared/read.rb
index e3840786d902..45d8d5bffbc7 100644
--- a/spec/ruby/library/stringio/shared/read.rb
+++ b/spec/ruby/library/stringio/shared/read.rb
@@ -11,10 +11,17 @@
   end
 
   it "reads length bytes and writes them to the buffer String" do
-    @io.send(@method, 7, buffer = +"")
+    @io.send(@method, 7, buffer = +"").should.equal?(buffer)
     buffer.should == "example"
   end
 
+  it "does not preserve the encoding of the given buffer" do
+    buffer = ''.encode(Encoding::ISO_8859_1)
+    @io.send(@method, 7, buffer)
+
+    buffer.encoding.should_not == Encoding::ISO_8859_1
+  end
+
   it "tries to convert the passed buffer Object to a String using #to_str" do
     obj = mock("to_str")
     obj.should_receive(:to_str).and_return(buffer = +"")
diff --git a/spec/tags/core/io/pread_tags.txt b/spec/tags/core/io/pread_tags.txt
index 22858b98bf63..84560c9368be 100644
--- a/spec/tags/core/io/pread_tags.txt
+++ b/spec/tags/core/io/pread_tags.txt
@@ -18,3 +18,4 @@ fails:IO#pread raises TypeError if offset is not an Integer and cannot be coerce
 fails:IO#pread raises ArgumentError for negative values of maxlen
 fails:IO#pread raised Errno::EINVAL for negative values of offset
 fails:IO#pread raises TypeError if the buffer is not a String and cannot be coerced into String
+fails:IO#pread preserves the encoding of the given buffer
diff --git a/spec/tags/core/io/read_nonblock_tags.txt b/spec/tags/core/io/read_nonblock_tags.txt
index 86717d874798..af86397479ae 100644
--- a/spec/tags/core/io/read_nonblock_tags.txt
+++ b/spec/tags/core/io/read_nonblock_tags.txt
@@ -1,3 +1,2 @@
 fails:IO#read_nonblock raises an exception after ungetc with data in the buffer and character conversion enabled
 fails:IO#read_nonblock discards the existing buffer content upon error
-fails:IO#read_nonblock preserves the encoding of the given buffer
diff --git a/spec/tags/core/io/readpartial_tags.txt b/spec/tags/core/io/readpartial_tags.txt
deleted file mode 100644
index fc74e20f7480..000000000000
--- a/spec/tags/core/io/readpartial_tags.txt
+++ /dev/null
@@ -1 +0,0 @@
-fails:IO#readpartial preserves the encoding of the given buffer
diff --git a/spec/tags/library/socket/basicsocket/read_nonblock_tags.txt b/spec/tags/library/socket/basicsocket/read_nonblock_tags.txt
index 97c413fb560d..ac56e3a0b03e 100644
--- a/spec/tags/library/socket/basicsocket/read_nonblock_tags.txt
+++ b/spec/tags/library/socket/basicsocket/read_nonblock_tags.txt
@@ -1,4 +1,2 @@
 fails:BasicSocket#read_nonblock using IPv4 does not set the IO in nonblock mode
 fails:BasicSocket#read_nonblock using IPv6 does not set the IO in nonblock mode
-fails:BasicSocket#read_nonblock using IPv4 replaces the content of the provided buffer without changing its encoding
-fails:BasicSocket#read_nonblock using IPv6 replaces the content of the provided buffer without changing its encoding
diff --git a/spec/tags/library/socket/basicsocket/recv_nonblock_tags.txt b/spec/tags/library/socket/basicsocket/recv_nonblock_tags.txt
deleted file mode 100644
index 9185207dafbc..000000000000
--- a/spec/tags/library/socket/basicsocket/recv_nonblock_tags.txt
+++ /dev/null
@@ -1,2 +0,0 @@
-fails:Socket::BasicSocket#recv_nonblock using IPv4 allows an output buffer as third argument
-fails:Socket::BasicSocket#recv_nonblock using IPv6 allows an output buffer as third argument
diff --git a/spec/tags/library/socket/basicsocket/recv_tags.txt b/spec/tags/library/socket/basicsocket/recv_tags.txt
deleted file mode 100644
index a51e691a9784..000000000000
--- a/spec/tags/library/socket/basicsocket/recv_tags.txt
+++ /dev/null
@@ -1 +0,0 @@
-fails:BasicSocket#recv allows an output buffer as third argument
diff --git a/spec/tags/library/socket/socket/recvfrom_nonblock_tags.txt b/spec/tags/library/socket/socket/recvfrom_nonblock_tags.txt
deleted file mode 100644
index 3a19e38dd791..000000000000
--- a/spec/tags/library/socket/socket/recvfrom_nonblock_tags.txt
+++ /dev/null
@@ -1 +0,0 @@
-fails:Socket#recvfrom_nonblock using IPv6 using a bound socket with data available the returned data is the same as the sent data
diff --git a/src/main/ruby/truffleruby/core/io.rb b/src/main/ruby/truffleruby/core/io.rb
index 213a9ea094c5..eec725b9d0ca 100644
--- a/src/main/ruby/truffleruby/core/io.rb
+++ b/src/main/ruby/truffleruby/core/io.rb
@@ -1673,6 +1673,8 @@ def read(length = nil, buffer = nil)
       str = IO.read_encode self, read_all
       return str unless buffer
 
+      # intentionally don't preserve buffer encoding
+      # see https://bugs.ruby-lang.org/issues/20416
       return buffer.replace(str)
     end
 
@@ -1753,10 +1755,14 @@ def read_nonblock(size, buffer = nil, exception: true)
     str = Truffle::POSIX.read_string_nonblock(self, size, exception)
 
     case str
-    when Symbol
+    when Symbol # :wait_readable in case of EAGAIN_ERRNO
       str
     when String
-      buffer ? buffer.replace(str) : str
+      if buffer
+        buffer.replace str.force_encoding(buffer.encoding)
+      else
+        str
+      end
     else # EOF
       if exception
         raise EOFError, 'end of file reached'
@@ -1871,8 +1877,7 @@ def readpartial(size, buffer = nil)
         raise EOFError if Primitive.nil? data
       end
 
-      buffer.replace(data)
-      buffer
+      buffer.replace data.force_encoding(buffer.encoding)
     else
       return +'' if size == 0
 
@@ -2196,20 +2201,23 @@ def sync=(v)
   #
   #  @todo  Improve reading into provided buffer.
   #
-  def sysread(number_of_bytes, buffer = undefined)
+  def sysread(number_of_bytes, buffer = nil)
     ensure_open_and_readable
     flush
     raise IOError unless @ibuffer.empty?
 
+    buffer = StringValue(buffer) if buffer
+
     str, errno = Truffle::POSIX.read_string(self, number_of_bytes)
     Errno.handle_errno(errno) unless errno == 0
 
     raise EOFError if Primitive.nil? str
 
-    unless Primitive.undefined? buffer
-      StringValue(buffer).replace str
+    if buffer
+      buffer.replace str.force_encoding(buffer.encoding)
+    else
+      str
     end
-    str
   end
 
   ##