diff --git a/core/src/main/java/org/jruby/RubyKernel.java b/core/src/main/java/org/jruby/RubyKernel.java
index 59fa811ee63..1952296dccd 100644
--- a/core/src/main/java/org/jruby/RubyKernel.java
+++ b/core/src/main/java/org/jruby/RubyKernel.java
@@ -127,6 +127,7 @@
 import static org.jruby.api.Create.*;
 import static org.jruby.api.Error.argumentError;
 import static org.jruby.api.Error.typeError;
+import static org.jruby.api.Warn.warnDeprecatedForRemovalAlternate;
 import static org.jruby.ir.runtime.IRRuntimeHelpers.dupIfKeywordRestAtCallsite;
 import static org.jruby.runtime.ThreadContext.hasKeywords;
 import static org.jruby.runtime.Visibility.PRIVATE;
@@ -294,6 +295,7 @@ public static IRubyObject open(ThreadContext context, IRubyObject recv, IRubyObj
                 } else {
                     IRubyObject cmd = PopenExecutor.checkPipeCommand(context, tmp);
                     if (cmd != context.nil) {
+                        warnDeprecatedForRemovalAlternate(context, "Calling Kernel#open with a leading '|'", "4.0", "IO.popen");
                         if (PopenExecutor.nativePopenAvailable(context.runtime)) {
                             args[0] = cmd;
                             return PopenExecutor.popen(context, args, ioClass(context), block);