diff --git a/lib/cf-socket.c b/lib/cf-socket.c
index a7c0ef0840..e3b3de4e1e 100644
--- a/lib/cf-socket.c
+++ b/lib/cf-socket.c
@@ -109,7 +109,7 @@ static void set_ipv6_v6only(curl_socket_t sockfd, int on)
 
 static void tcpnodelay(struct Curl_easy *data, curl_socket_t sockfd)
 {
-#if defined(TCP_NODELAY) && !defined(__EMSCRIPTEN__)
+#if defined(TCP_NODELAY) && defined(CURL_TCP_NODELAY_SUPPORTED)
   curl_socklen_t onoff = (curl_socklen_t) 1;
   int level = IPPROTO_TCP;
   char buffer[STRERROR_LEN];
diff --git a/lib/curl_setup.h b/lib/curl_setup.h
index 9b663fcb7a..0a6591db83 100644
--- a/lib/curl_setup.h
+++ b/lib/curl_setup.h
@@ -1125,6 +1125,11 @@ int getpwuid_r(uid_t uid, struct passwd *pwd, char *buf,
 #define USE_HTTP3
 #endif
 
+/* WebAssembly builds have TCP_NODELAY, but runtime support is missing. */
+#ifndef __EMSCRIPTEN__
+#define CURL_TCP_NODELAY_SUPPORTED
+#endif
+
 /* Certain Windows implementations are not aligned with what curl expects,
    so always use the local one on this platform. E.g. the mingw-w64
    implementation can return wrong results for non-ASCII inputs. */
diff --git a/tests/server/sws.c b/tests/server/sws.c
index f03b9bdcdb..2b94e52938 100644
--- a/tests/server/sws.c
+++ b/tests/server/sws.c
@@ -158,7 +158,7 @@ static const char *doc404 = "HTTP/1.1 404 Not Found\r\n"
 /* work around for handling trailing headers */
 static int already_recv_zeroed_chunk = FALSE;
 
-#if defined(TCP_NODELAY) && !defined(__EMSCRIPTEN__)
+#if defined(TCP_NODELAY) && defined(CURL_TCP_NODELAY_SUPPORTED)
 /* returns true if the current socket is an IP one */
 static bool socket_domain_is_ip(void)
 {
@@ -1239,7 +1239,7 @@ static curl_socket_t connect_to(const char *ipaddr, unsigned short port)
     return CURL_SOCKET_BAD;
   }
 
-#if defined(TCP_NODELAY) && !defined(__EMSCRIPTEN__)
+#if defined(TCP_NODELAY) && defined(CURL_TCP_NODELAY_SUPPORTED)
   if(socket_domain_is_ip()) {
     /* Disable the Nagle algorithm */
     curl_socklen_t flag = 1;
@@ -1545,7 +1545,7 @@ static void http_connect(curl_socket_t *infdp,
           }
           memset(req2, 0, sizeof(*req2));
           logmsg("====> Client connect DATA");
-#if defined(TCP_NODELAY) && !defined(__EMSCRIPTEN__)
+#if defined(TCP_NODELAY) && defined(CURL_TCP_NODELAY_SUPPORTED)
           if(socket_domain_is_ip()) {
             /* Disable the Nagle algorithm */
             curl_socklen_t flag = 1;
@@ -1871,7 +1871,7 @@ static curl_socket_t accept_connection(curl_socket_t sock)
   all_sockets[num_sockets] = msgsock;
   num_sockets += 1;
 
-#if defined(TCP_NODELAY) && !defined(__EMSCRIPTEN__)
+#if defined(TCP_NODELAY) && defined(CURL_TCP_NODELAY_SUPPORTED)
   if(socket_domain_is_ip()) {
     /*
      * Disable the Nagle algorithm to make it easier to send out a large