diff --git a/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/HttpMessageParser.scala b/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/HttpMessageParser.scala
index fbdc2d664..3080aa834 100644
--- a/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/HttpMessageParser.scala
+++ b/http-core/src/main/scala/org/apache/pekko/http/impl/engine/parsing/HttpMessageParser.scala
@@ -204,6 +204,13 @@ private[http] trait HttpMessageParser[Output >: MessageOutput <: ParserOutput] {
             // only allow one 'chunked'
             failMessageStart("Multiple Transfer-Encoding entries not supported")
           }
+
+        // `transfer-encoding` is always modelled (it is in `alwaysParsedHeaders`), so it only reaches us as a
+        // RawHeader when its value failed to parse and was degraded to one. Framing must not silently fall back to
+        // Content-Length then: an upstream that does understand the value would frame the message differently, which
+        // is a request smuggling discrepancy.
+        case h: RawHeader if h.lowercaseName == "transfer-encoding" =>
+          failMessageStart("Illegal `Transfer-Encoding` header value")
         case h: Connection => ch match {
             case None =>
               parseHeaderLines(input, lineEnd, headers += h, headerCount + 1, Some(h), clh, cth, isChunked, e100c, hh)
diff --git a/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala b/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala
index d346f045f..7edbc1526 100644
--- a/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala
+++ b/http-core/src/test/scala/org/apache/pekko/http/impl/engine/parsing/RequestParserSpec.scala
@@ -708,6 +708,17 @@ abstract class RequestParserSpec(mode: String, newLine: String) extends AnyFreeS
           ErrorInfo("HTTP message contains more than the configured limit of 2 headers"))
       }
 
+      "with an unparseable Transfer-Encoding header value" in new Test {
+        // the value cannot be modelled, so it would otherwise degrade to a RawHeader and the message would be framed
+        // by Content-Length while an upstream that does understand it frames by chunked encoding
+        """POST / HTTP/1.1
+          |Host: x
+          |Transfer-Encoding: "chunked"
+          |Content-Length: 3
+          |
+          |abc""" should parseToError(BadRequest, ErrorInfo("Illegal `Transfer-Encoding` header value"))
+      }
+
       "with an invalid Content-Length header value" in new Test {
         """GET / HTTP/1.0
           |Content-Length: 1.5