diff --git a/cli/js/lib.deno.ns.d.ts b/cli/js/lib.deno.ns.d.ts
index 8ae71b23f6..d33fde8449 100644
--- a/cli/js/lib.deno.ns.d.ts
+++ b/cli/js/lib.deno.ns.d.ts
@@ -2177,6 +2177,13 @@ declare namespace Deno {
      * This calls `close()` on stderr after its done. */
     stderrOutput(): Promise<Uint8Array>;
     close(): void;
+
+    /** **UNSTABLE**: The `signo` argument may change to require the Deno.Signal
+     * enum.
+     *
+     * Send a signal to process. This functionality currently only works on
+     * Linux and Mac OS.
+     */
     kill(signo: number): void;
   }
 
diff --git a/cli/ops/process.rs b/cli/ops/process.rs
index 0384f5b39d..125aa136b4 100644
--- a/cli/ops/process.rs
+++ b/cli/ops/process.rs
@@ -229,6 +229,7 @@ fn op_kill(
   args: Value,
   _zero_copy: Option<ZeroCopyBuf>,
 ) -> Result<JsonOp, OpError> {
+  state.check_unstable("Deno.kill");
   state.check_run()?;
 
   let args: KillArgs = serde_json::from_value(args)?;