diff --git a/curl-sys/build.rs b/curl-sys/build.rs
index 7d028d410..82348e251 100644
--- a/curl-sys/build.rs
+++ b/curl-sys/build.rs
@@ -304,15 +304,14 @@ fn main() {
         .warnings(false);
 
     if cfg!(feature = "ntlm") {
-        cfg.file("curl/lib/curl_endian.c")
+        cfg.define("CURL_ENABLE_NTLM", None)
+            .file("curl/lib/curl_endian.c")
             .file("curl/lib/curl_gethostname.c")
             .file("curl/lib/curl_ntlm_core.c")
             .file("curl/lib/http_ntlm.c")
             .file("curl/lib/md4.c")
             .file("curl/lib/vauth/ntlm.c")
             .file("curl/lib/vauth/ntlm_sspi.c");
-    } else {
-        cfg.define("CURL_DISABLE_NTLM", None);
     }
 
     if cfg!(feature = "protocol-ftp") {
diff --git a/src/lib.rs b/src/lib.rs
index 2965e2bed..09aa757dc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -181,4 +181,14 @@ mod tests {
     fn is_initialized_before_main() {
         assert!(INITIALIZED.load(std::sync::atomic::Ordering::SeqCst));
     }
+
+    /// Regression test for the libcurl 8.20 `CURL_ENABLE_NTLM` opt-in flip.
+    #[cfg(feature = "ntlm")]
+    #[test]
+    fn is_ntlm_support_activated() {
+        assert!(
+            Version::get().feature_ntlm(),
+            "`CURL_ENABLE_NTLM` may not be defined"
+        );
+    }
 }