diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index 36fd327d4b9..89ac680efd5 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -270,6 +270,18 @@
      </thead>
 
      <tbody>
+      <row>
+      <entry>3.9999</entry>
+      <entry>-</entry>
+      <entry>Reserved for protocol greasing. libpq may use this version, which
+        is higher than any minor version the project ever expects to use, to
+        test that servers and middleware properly implement protocol version
+        negotiation. Servers <emphasis>must not</emphasis> add special-case
+        logic for this version; they should simply compare it to their latest
+        supported version (which will always be smaller) and downgrade via a
+        NegotiateProtocolVersion message.
+      </entry>
+      </row>
       <row>
       <entry>3.1</entry>
       <entry>-</entry>
@@ -353,6 +365,17 @@
         otherwise continue the connection.
       </entry>
       </row>
+
+      <row>
+      <entry><literal>_pq_.test_protocol_negotiation</literal></entry>
+      <entry>Reserved for protocol greasing. libpq may send this extension to
+        test that servers and middleware properly implement protocol extension
+        negotiation. Servers <emphasis>must not</emphasis> add special-case
+        logic for this parameter; they should simply send the list of all
+        unsupported options (including this one) via a NegotiateProtocolVersion
+        message.
+      </entry>
+      </row>
      </tbody>
     </tgroup>
    </table>
diff --git a/src/include/libpq/pqcomm.h b/src/include/libpq/pqcomm.h
index 1bbe5b9ee45..a29c9c94d79 100644
--- a/src/include/libpq/pqcomm.h
+++ b/src/include/libpq/pqcomm.h
@@ -104,6 +104,16 @@ is_unixsock_path(const char *path)
  */
 #define PG_PROTOCOL_RESERVED_31		PG_PROTOCOL(3,1)
 
+/*
+ * PG_PROTOCOL_GREASE is an intentionally unsupported protocol version used
+ * for "greasing" (the practice of sending valid, but extraneous or otherwise
+ * unusual, messages to keep peer implementations honest). This helps ensure
+ * that servers properly implement protocol version negotiation. Version 3.9999
+ * was chosen since it is safely within the valid range, it is representable
+ * via PQfullProtocolVersion, and it is unlikely to ever be needed in practice.
+ */
+#define PG_PROTOCOL_GREASE		PG_PROTOCOL(3,9999)
+
 /*
  * A client can send a cancel-current-operation request to the postmaster.
  * This is uglier than sending it directly to the client's backend, but it
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 103428033ef..90bbb2eba1f 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -1451,7 +1451,19 @@ pqGetNegotiateProtocolVersion3(PGconn *conn)
 	if (pqGetInt(&num, 4, conn) != 0)
 		goto eof;
 
-	/* Check the protocol version */
+	/*
+	 * Check the protocol version.
+	 *
+	 * PG_PROTOCOL_GREASE is intentionally unsupported and reserved. It's
+	 * higher than any real version, so check for that first, to get the most
+	 * specific error message. Then check the upper and lower bounds.
+	 */
+	if (their_version == PG_PROTOCOL_GREASE)
+	{
+		libpq_append_conn_error(conn, "received invalid protocol negotiation message: server requested \"grease\" protocol version 3.9999");
+		goto failure;
+	}
+
 	if (their_version > conn->pversion)
 	{
 		libpq_append_conn_error(conn, "received invalid protocol negotiation message: server requested downgrade to a higher-numbered version");