diff --git a/prober/grpc.go b/prober/grpc.go
index cd43c711..a73b6d19 100644
--- a/prober/grpc.go
+++ b/prober/grpc.go
@@ -109,7 +109,7 @@ func ProbeGRPC(ctx context.Context, target string, module config.Module, registr
 				Name: "probe_ssl_last_chain_info",
 				Help: "Contains SSL leaf certificate information",
 			},
-			[]string{"fingerprint_sha256", "subject", "issuer", "subjectalternative"},
+			[]string{"fingerprint_sha256", "subject", "issuer", "subjectalternative", "serialnumber"},
 		)
 	)
 
@@ -204,7 +204,7 @@ func ProbeGRPC(ctx context.Context, target string, module config.Module, registr
 			isSSLGauge.Set(float64(1))
 			probeSSLEarliestCertExpiryGauge.Set(float64(getEarliestCertExpiry(&tlsInfo.State).Unix()))
 			probeTLSVersion.WithLabelValues(getTLSVersion(&tlsInfo.State)).Set(1)
-			probeSSLLastInformation.WithLabelValues(getFingerprint(&tlsInfo.State), getSubject(&tlsInfo.State), getIssuer(&tlsInfo.State), getDNSNames(&tlsInfo.State)).Set(1)
+			probeSSLLastInformation.WithLabelValues(getFingerprint(&tlsInfo.State), getSubject(&tlsInfo.State), getIssuer(&tlsInfo.State), getDNSNames(&tlsInfo.State), getSerialNumber(&tlsInfo.State)).Set(1)
 		} else {
 			isSSLGauge.Set(float64(0))
 		}
diff --git a/prober/http.go b/prober/http.go
index 5ffc029f..864bf93b 100644
--- a/prober/http.go
+++ b/prober/http.go
@@ -273,7 +273,7 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 				Name: "probe_ssl_last_chain_info",
 				Help: "Contains SSL leaf certificate information",
 			},
-			[]string{"fingerprint_sha256", "subject", "issuer", "subjectalternative"},
+			[]string{"fingerprint_sha256", "subject", "issuer", "subjectalternative", "serialnumber"},
 		)
 
 		probeTLSVersion = prometheus.NewGaugeVec(
@@ -647,7 +647,7 @@ func ProbeHTTP(ctx context.Context, target string, module config.Module, registr
 		probeTLSVersion.WithLabelValues(getTLSVersion(resp.TLS)).Set(1)
 		probeTLSCipher.WithLabelValues(getTLSCipher(resp.TLS)).Set(1)
 		probeSSLLastChainExpiryTimestampSeconds.Set(float64(getLastChainExpiry(resp.TLS).Unix()))
-		probeSSLLastInformation.WithLabelValues(getFingerprint(resp.TLS), getSubject(resp.TLS), getIssuer(resp.TLS), getDNSNames(resp.TLS)).Set(1)
+		probeSSLLastInformation.WithLabelValues(getFingerprint(resp.TLS), getSubject(resp.TLS), getIssuer(resp.TLS), getDNSNames(resp.TLS), getSerialNumber(resp.TLS)).Set(1)
 		if httpConfig.FailIfSSL {
 			logger.Error("Final request was over SSL")
 			success = false
diff --git a/prober/tcp.go b/prober/tcp.go
index 98262813..b10c0b5d 100644
--- a/prober/tcp.go
+++ b/prober/tcp.go
@@ -113,7 +113,7 @@ func ProbeTCP(ctx context.Context, target string, module config.Module, registry
 			Name: "probe_ssl_last_chain_info",
 			Help: "Contains SSL leaf certificate information",
 		},
-		[]string{"fingerprint_sha256", "subject", "issuer", "subjectalternative"},
+		[]string{"fingerprint_sha256", "subject", "issuer", "subjectalternative", "serialnumber"},
 	)
 	probeTLSVersion := prometheus.NewGaugeVec(
 		probeTLSInfoGaugeOpts,
@@ -147,7 +147,7 @@ func ProbeTCP(ctx context.Context, target string, module config.Module, registry
 		probeSSLEarliestCertExpiry.Set(float64(getEarliestCertExpiry(&state).Unix()))
 		probeTLSVersion.WithLabelValues(getTLSVersion(&state)).Set(1)
 		probeSSLLastChainExpiryTimestampSeconds.Set(float64(getLastChainExpiry(&state).Unix()))
-		probeSSLLastInformation.WithLabelValues(getFingerprint(&state), getSubject(&state), getIssuer(&state), getDNSNames(&state)).Set(1)
+		probeSSLLastInformation.WithLabelValues(getFingerprint(&state), getSubject(&state), getIssuer(&state), getDNSNames(&state), getSerialNumber(&state)).Set(1)
 	}
 	scanner := bufio.NewScanner(conn)
 	for i, qr := range module.TCP.QueryResponse {
@@ -216,7 +216,7 @@ func ProbeTCP(ctx context.Context, target string, module config.Module, registry
 			probeSSLEarliestCertExpiry.Set(float64(getEarliestCertExpiry(&state).Unix()))
 			probeTLSVersion.WithLabelValues(getTLSVersion(&state)).Set(1)
 			probeSSLLastChainExpiryTimestampSeconds.Set(float64(getLastChainExpiry(&state).Unix()))
-			probeSSLLastInformation.WithLabelValues(getFingerprint(&state), getSubject(&state), getIssuer(&state), getDNSNames(&state)).Set(1)
+			probeSSLLastInformation.WithLabelValues(getFingerprint(&state), getSubject(&state), getIssuer(&state), getDNSNames(&state), getSerialNumber(&state)).Set(1)
 		}
 	}
 	return true
diff --git a/prober/tls.go b/prober/tls.go
index 3da17a05..0589036e 100644
--- a/prober/tls.go
+++ b/prober/tls.go
@@ -17,6 +17,7 @@ import (
 	"crypto/sha256"
 	"crypto/tls"
 	"encoding/hex"
+	"fmt"
 	"strings"
 	"time"
 )
@@ -69,6 +70,14 @@ func getLastChainExpiry(state *tls.ConnectionState) time.Time {
 	return lastChainExpiry
 }
 
+func getSerialNumber(state *tls.ConnectionState) string {
+	cert := state.PeerCertificates[0]
+	// Using `cert.SerialNumber.Text(16)` will drop the leading zeros when converting the SerialNumber to String, see https://github.com/mozilla/tls-observatory/pull/245.
+	// To avoid that, we format in lowercase the bytes with `%x` to base 16, with lower-case letters for a-f, see https://go.dev/play/p/Fylce70N2Zl.
+
+	return fmt.Sprintf("%x", cert.SerialNumber.Bytes())
+}
+
 func getTLSVersion(state *tls.ConnectionState) string {
 	switch state.Version {
 	case tls.VersionTLS10:
diff --git a/prober/utils_test.go b/prober/utils_test.go
index cbc21210..bc85d23d 100644
--- a/prober/utils_test.go
+++ b/prober/utils_test.go
@@ -17,6 +17,7 @@ import (
 	"context"
 	"crypto/rand"
 	"crypto/rsa"
+	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/pem"
@@ -251,6 +252,46 @@ func checkMetrics(expected map[string]map[string]map[string]struct{}, mfs []*dto
 	}
 }
 
+func TestGetSerialNumber(t *testing.T) {
+	tests := []struct {
+		name         string
+		serialNumber *big.Int
+		expected     string
+	}{
+		{
+			name: "Serial number with leading zeros",
+			serialNumber: func() *big.Int {
+				serialNumber, _ := new(big.Int).SetString("0BFFBC11F1907D02AF719AFCD64FB253", 16)
+				return serialNumber
+			}(),
+			expected: "0bffbc11f1907d02af719afcd64fb253",
+		},
+		{
+			name: "Serial number without leading zeros",
+			serialNumber: func() *big.Int {
+				serialNumber, _ := new(big.Int).SetString("BBFFBC11F1907D02AF719AFCD64FB253", 16)
+				return serialNumber
+			}(),
+			expected: "bbffbc11f1907d02af719afcd64fb253",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cert := &x509.Certificate{
+				SerialNumber: tt.serialNumber,
+			}
+			state := &tls.ConnectionState{
+				PeerCertificates: []*x509.Certificate{cert},
+			}
+			result := getSerialNumber(state)
+			if result != tt.expected {
+				t.Errorf("expected %s, got %s", tt.expected, result)
+			}
+		})
+	}
+}
+
 func checkAbsentMetrics(absent []string, mfs []*dto.MetricFamily, t *testing.T) {
 	for _, v := range mfs {
 		name := v.GetName()