diff --git a/doc/NEWS.Rd b/doc/NEWS.Rd
index 9ffc4b683c..e030cdb768 100644
--- a/doc/NEWS.Rd
+++ b/doc/NEWS.Rd
@@ -144,6 +144,10 @@
       \item If \code{La_library()} is empty, \code{sessionInfo()} still
       reports \code{La_version()} when available.
 
+      \item \code{seq.int(from, to, by, ....)} when \eqn{|by| = 1} now
+      behaves as if \code{by} was omitted, and hence returns \code{from:to},
+      possibly integer.
+
       \item \code{seq.Date(from, to, by, ....)} and \code{seq.POSIXt(..)}
       now also work when \code{from} is missing and sufficient further
       arguments are provided, thanks to \I{Michael Chirico}'s report, patch
diff --git a/src/main/seq.c b/src/main/seq.c
index d4387a7e40..51cc2167da 100644
--- a/src/main/seq.c
+++ b/src/main/seq.c
@@ -1,6 +1,6 @@
 /*
  *  R : A Computer Language for Statistical Data Analysis
- *  Copyright (C) 1998-2023  The R Core Team.
+ *  Copyright (C) 1998-2025  The R Core Team.
  *  Copyright (C) 1995-1998  Robert Gentleman and Ross Ihaka
  *
  *  This program is free software; you can redistribute it and/or modify
@@ -109,7 +109,7 @@ static SEXP seq_colon(double n1, double n2, SEXP call)
 
     Rboolean useInt = (n1 <= INT_MAX) &&  (n1 == (int) n1);
     if(useInt) {
-	if(n1 <= INT_MIN || n1 > INT_MAX)
+	if(n1 <= INT_MIN) /* know  n1 <= INT_MAX */
 	    useInt = FALSE;
 	else {
 	    /* r := " the effective 'to' "  of  from:to */
@@ -787,11 +787,12 @@ attribute_hidden SEXP do_rep(SEXP call, SEXP op, SEXP args, SEXP rho)
 
 
 /*
-  This is a primitive SPECIALSXP with internal argument matching,
-  implementing seq.int().
+  This is a primitive SPECIALSXP with internal argument matching, implementing
 
-   'along' has to be used on an unevaluated argument, and evalList
-   tries to evaluate language objects.
+  seq.int(from, to, by, length.out, along.with, ...)
+
+  'along' has to be used on an unevaluated argument, and evalList
+  tries to evaluate language objects.
  */
 #define FEPS 1e-10
 /* to match seq.default */
@@ -832,7 +833,7 @@ attribute_hidden SEXP do_seq(SEXP call, SEXP op, SEXP args, SEXP rho)
 		errorcall(call, _("'%s' must be a finite number"), "from");
 	    ans = seq_colon(1.0, rfrom, call);
 	}
-	else if (lf)
+	else if (lf) // typically  seq(<vec>) , length(<vec>) >= 2
 	    ans = seq_colon(1.0, (double)lf, call);
 	else
 	    ans = allocVector(INTSXP, 0);
@@ -884,13 +885,16 @@ attribute_hidden SEXP do_seq(SEXP call, SEXP op, SEXP args, SEXP rho)
 		ans = to; // is *not* missing in this case
 		goto done;
 	    }
-	    double n, rby = asReal(by);
-	    Rboolean finite_del = R_FINITE(del);
-	    if(finite_del) {
-		n = del/rby;
-	    } else { // overflow in  (to - from)  when both are finite
-		n = rto/rby - rfrom/rby;
+	    double rby = asReal(by);
+	    if((rby ==  1. && del > 0.) ||
+	       (rby == -1. && del < 0.)) { // --> treat as if missing (return integer)
+		ans = seq_colon(rfrom, rto, call);
+		goto done;
 	    }
+	    Rboolean finite_del = R_FINITE(del);
+	    double n = (finite_del)
+		? del/rby
+		: rto/rby - rfrom/rby; /* overflow in  (to - from)  when both are finite */
 	    if(!R_FINITE(n)) {
 		if(del == 0.0 && rby == 0.0) {
 		    ans = miss_from ? ScalarReal(rfrom) : from;
@@ -1047,7 +1051,7 @@ attribute_hidden SEXP do_seq(SEXP call, SEXP op, SEXP args, SEXP rho)
 done:
     UNPROTECT(1);
     return ans;
-}
+} // do_seq()
 
 attribute_hidden SEXP do_seq_along(SEXP call, SEXP op, SEXP args, SEXP rho)
 {
diff --git a/tests/reg-tests-1d.R b/tests/reg-tests-1d.R
index d67f0ada07..d6800b5a28 100644
--- a/tests/reg-tests-1d.R
+++ b/tests/reg-tests-1d.R
@@ -644,7 +644,7 @@ stopifnot(exprs = {
 ## had failed in R-devel for a few days
 D1 <- as.Date("2017-01-06")
 D2 <- as.Date("2017-01-12")
-seqD1 <- seq.Date(D1, D2, by = "1 day")
+seqD1 <- seq(D1, D2, by = "1 day")
 stopifnot(exprs = {
     identical(seqD1, seq(D1, D2)) # by = "days" implicit default since R >= 4.5
     identical(seqD1, seq(D1, D2, by = "1 days"))