diff --git a/src/http/client.cr b/src/http/client.cr
index a819741b728b..2fb0cd9d7c45 100644
--- a/src/http/client.cr
+++ b/src/http/client.cr
@@ -58,6 +58,8 @@
 # client.close
 # ```
 #
+# WARNING: A single `HTTP::Client` instance is not safe for concurrent use by multiple fibers.
+#
 # ### Compression
 #
 # If `compress` isn't set to `false`, and no `Accept-Encoding` header is explicitly specified,