--- a/src/backend/libpq/be-secure-openssl.c	2025-02-17 16:14:22.000000000 -0500
+++ b/src/backend/libpq/be-secure-openssl.c	2026-03-30 14:33:40.542000000 -0400
@@ -46,9 +46,6 @@
 #include "common/openssl.h"
 #include <openssl/conf.h>
 #include <openssl/dh.h>
-#ifndef OPENSSL_NO_ECDH
-#include <openssl/ec.h>
-#endif
 #include <openssl/x509v3.h>
 
 
@@ -69,6 +66,7 @@
 static void info_cb(const SSL *ssl, int type, int args);
 static bool initialize_dh(SSL_CTX *context, bool isServerStart);
 static bool initialize_ecdh(SSL_CTX *context, bool isServerStart);
+static const char *SSLerrmessageExt(unsigned long ecode, const char *replacement);
 static const char *SSLerrmessage(unsigned long ecode);
 
 static char *X509_NAME_to_cstring(X509_NAME *name);
@@ -1320,41 +1318,55 @@
  * Set ECDH parameters for generating ephemeral Elliptic Curve DH
  * keys.  This is much simpler than the DH parameters, as we just
  * need to provide the name of the curve to OpenSSL.
+ *
+ * Unlike the pre-PG18 implementation that used OBJ_sn2nid() and
+ * EC_KEY_new_by_curve_name() which only accepted a single curve,
+ * SSL_CTX_set1_groups_list() accepts a colon-separated list of
+ * group names, enabling post-quantum hybrid key exchange groups
+ * such as X25519MLKEM768.
  */
 static bool
 initialize_ecdh(SSL_CTX *context, bool isServerStart)
 {
 #ifndef OPENSSL_NO_ECDH
-	EC_KEY	   *ecdh;
-	int			nid;
-
-	nid = OBJ_sn2nid(SSLECDHCurve);
-	if (!nid)
-	{
-		ereport(isServerStart ? FATAL : LOG,
-				(errcode(ERRCODE_CONFIG_FILE_ERROR),
-				 errmsg("ECDH: unrecognized curve name: %s", SSLECDHCurve)));
-		return false;
-	}
-
-	ecdh = EC_KEY_new_by_curve_name(nid);
-	if (!ecdh)
+	if (SSL_CTX_set1_groups_list(context, SSLECDHCurve) != 1)
 	{
 		ereport(isServerStart ? FATAL : LOG,
 				(errcode(ERRCODE_CONFIG_FILE_ERROR),
-				 errmsg("ECDH: could not create key")));
+				 errmsg("ECDH: could not set group names from ssl_ecdh_curve: %s",
+						SSLerrmessageExt(ERR_get_error(),
+										 _("No valid groups found"))),
+				 errhint("Ensure that each group name is spelled correctly and supported by the installed version of OpenSSL.")));
 		return false;
 	}
-
-	SSL_CTX_set_options(context, SSL_OP_SINGLE_ECDH_USE);
-	SSL_CTX_set_tmp_ecdh(context, ecdh);
-	EC_KEY_free(ecdh);
 #endif
 
 	return true;
 }
 
 /*
+ * Obtain reason string for passed SSL errcode with replacement
+ *
+ * The error message supplied in replacement will be used in case the error
+ * code from OpenSSL is 0, else the error message from SSLerrmessage() will
+ * be returned.
+ *
+ * Not all versions of OpenSSL place an error on the queue even for failing
+ * operations, which will yield "no SSL error reported" by SSLerrmessage.
+ * This function can be used to ensure that a proper error message is displayed
+ * for versions reporting no error, while using the OpenSSL error via
+ * SSLerrmessage for versions where there is one.
+ */
+static const char *
+SSLerrmessageExt(unsigned long ecode, const char *replacement)
+{
+	if (ecode == 0)
+		return replacement;
+	else
+		return SSLerrmessage(ecode);
+}
+
+/*
  * Obtain reason string for passed SSL errcode
  *
  * ERR_get_error() is used by caller to get errcode to pass here.
--- a/src/backend/utils/misc/guc_tables.c	2025-02-17 16:14:22.000000000 -0500
+++ b/src/backend/utils/misc/guc_tables.c	2026-03-30 14:32:36.585000000 -0400
@@ -4449,8 +4449,8 @@
 
 	{
 		{"ssl_ecdh_curve", PGC_SIGHUP, CONN_AUTH_SSL,
-			gettext_noop("Sets the curve to use for ECDH."),
-			NULL,
+			gettext_noop("Sets the group(s) to use for key exchange."),
+			gettext_noop("Multiple groups can be specified using a colon-separated list."),
 			GUC_SUPERUSER_ONLY
 		},
 		&SSLECDHCurve,
--- a/src/backend/utils/misc/postgresql.conf.sample	2025-02-17 16:14:22.000000000 -0500
+++ b/src/backend/utils/misc/postgresql.conf.sample	2026-03-30 14:32:36.587000000 -0400
@@ -113,7 +113,7 @@
 #ssl_key_file = 'server.key'
 #ssl_ciphers = 'HIGH:MEDIUM:+3DES:!aNULL' # allowed SSL ciphers
 #ssl_prefer_server_ciphers = on
-#ssl_ecdh_curve = 'prime256v1'
+#ssl_ecdh_curve = 'prime256v1'		# colon-separated list of group names
 #ssl_min_protocol_version = 'TLSv1.2'
 #ssl_max_protocol_version = ''
 #ssl_dh_params_file = ''