diff --git a/CHANGELOG.next.asciidoc b/CHANGELOG.next.asciidoc
index 0e1ef392c892..f70d0d53b1d3 100644
--- a/CHANGELOG.next.asciidoc
+++ b/CHANGELOG.next.asciidoc
@@ -91,6 +91,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Change cache processor documentation from `write_period` to `write_interval`. {pull}38561[38561]
 - Fix cache processor expiries heap cleanup on partial file writes. {pull}38561[38561]
 - Fix cache processor expiries infinite growth when large a large TTL is used and recurring keys are cached. {pull}38561[38561]
+- Fix parsing of RFC 3164 process IDs in syslog processor. {issue}38947[38947] {pull}38982[38982]
 
 *Auditbeat*
 - Set field types to correctly match ECS in sessionmd processor {issue}38955[38955] {pull}38994[38994]
diff --git a/libbeat/reader/syslog/parser/rfc3164.rl b/libbeat/reader/syslog/parser/rfc3164.rl
index 0dac77314040..709c049eb06e 100644
--- a/libbeat/reader/syslog/parser/rfc3164.rl
+++ b/libbeat/reader/syslog/parser/rfc3164.rl
@@ -16,7 +16,7 @@
     hostname  = graph+ >tok %set_hostname;
 
     tag           = (print -- [ :\[])+ >tok %set_tag;
-    content_value = print+ >tok %set_content;
+    content_value = digit+ >tok %set_content;
     content       = '[' content_value ']';
     msg           = (tag content? ':' sp)? any+ >tok %set_msg;
 }%%
diff --git a/libbeat/reader/syslog/rfc3164_gen.go b/libbeat/reader/syslog/rfc3164_gen.go
index 852ec066f597..b4c4146ba82b 100644
--- a/libbeat/reader/syslog/rfc3164_gen.go
+++ b/libbeat/reader/syslog/rfc3164_gen.go
@@ -80,10 +80,6 @@ func parseRFC3164(data string, loc *time.Location) (message, error) {
 			goto st_case_29
 		case 30:
 			goto st_case_30
-		case 31:
-			goto st_case_31
-		case 32:
-			goto st_case_32
 		case 7:
 			goto st_case_7
 		case 8:
@@ -317,7 +313,7 @@ func parseRFC3164(data string, loc *time.Location) (message, error) {
 			goto _test_eof28
 		}
 	st_case_28:
-		if 32 <= data[p] && data[p] <= 126 {
+		if 48 <= data[p] && data[p] <= 57 {
 			goto tr37
 		}
 		goto st24
@@ -334,7 +330,7 @@ func parseRFC3164(data string, loc *time.Location) (message, error) {
 		if data[p] == 93 {
 			goto tr39
 		}
-		if 32 <= data[p] && data[p] <= 126 {
+		if 48 <= data[p] && data[p] <= 57 {
 			goto st29
 		}
 		goto st24
@@ -342,56 +338,16 @@ func parseRFC3164(data string, loc *time.Location) (message, error) {
 
 		m.setContent(data[tok:p])
 
-		goto st30
-	tr42:
-
-		m.setContent(data[tok:p])
-
-		tok = p
-
 		goto st30
 	st30:
 		if p++; p == pe {
 			goto _test_eof30
 		}
 	st_case_30:
-		switch data[p] {
-		case 58:
-			goto st31
-		case 93:
-			goto tr39
-		}
-		if 32 <= data[p] && data[p] <= 126 {
-			goto st29
-		}
-		goto st24
-	st31:
-		if p++; p == pe {
-			goto _test_eof31
-		}
-	st_case_31:
-		switch data[p] {
-		case 32:
-			goto st32
-		case 93:
-			goto tr39
-		}
-		if 33 <= data[p] && data[p] <= 126 {
-			goto st29
+		if data[p] == 58 {
+			goto st26
 		}
 		goto st24
-	st32:
-		if p++; p == pe {
-			goto _test_eof32
-		}
-	st_case_32:
-		if data[p] == 93 {
-			goto tr42
-		}
-		if 32 <= data[p] && data[p] <= 126 {
-			goto tr37
-		}
-		goto tr11
 	st7:
 		if p++; p == pe {
 			goto _test_eof7
@@ -816,12 +772,6 @@ func parseRFC3164(data string, loc *time.Location) (message, error) {
 	_test_eof30:
 		cs = 30
 		goto _test_eof
-	_test_eof31:
-		cs = 31
-		goto _test_eof
-	_test_eof32:
-		cs = 32
-		goto _test_eof
 	_test_eof7:
 		cs = 7
 		goto _test_eof
@@ -879,7 +829,7 @@ func parseRFC3164(data string, loc *time.Location) (message, error) {
 		}
 		if p == eof {
 			switch cs {
-			case 24, 25, 26, 27, 28, 29, 30, 31, 32:
+			case 24, 25, 26, 27, 28, 29, 30:
 
 				m.setMsg(data[tok:p])
 
diff --git a/libbeat/reader/syslog/rfc3164_test.go b/libbeat/reader/syslog/rfc3164_test.go
index d1c75fe574eb..a2c80e8c263f 100644
--- a/libbeat/reader/syslog/rfc3164_test.go
+++ b/libbeat/reader/syslog/rfc3164_test.go
@@ -88,6 +88,19 @@ func TestParseRFC3164(t *testing.T) {
 				msg:       "message",
 			},
 		},
+		"ok-procid-with-square-brackets-msg": {
+			in: "<114>Apr 12 13:30:01 aaaaaa001.adm.domain aaaaaa001[25259]: my.some.domain 10.11.12.13 - USERNAME [12/Apr/2024:13:29:59.993 +0200] /skodas \"GET /skodas/group/pod-documentation/aaa HTTP/1.1\" 301 301 290bytes 1 10327",
+			want: message{
+				timestamp: mustParseTime(time.Stamp, "Apr 12 13:30:01", time.Local),
+				priority:  114,
+				facility:  14,
+				severity:  2,
+				hostname:  "aaaaaa001.adm.domain",
+				process:   "aaaaaa001",
+				pid:       "25259",
+				msg:       "my.some.domain 10.11.12.13 - USERNAME [12/Apr/2024:13:29:59.993 +0200] /skodas \"GET /skodas/group/pod-documentation/aaa HTTP/1.1\" 301 301 290bytes 1 10327",
+			},
+		},
 		"err-pri-not-a-number": {
 			in: "<abc>Oct 11 22:14:15 test-host this is the message",
 			want: message{