diff --git a/types/meow/index.d.ts b/types/meow/index.d.ts
index db3d697691..0ba420bf04 100644
--- a/types/meow/index.d.ts
+++ b/types/meow/index.d.ts
@@ -1,4 +1,4 @@
-// Type definitions for meow 4.x
+// Type definitions for meow 5.x
 // Project: https://github.com/sindresorhus/meow
 // Definitions by: KnisterPeter <https://github.com/KnisterPeter>
 //                 Lindsey Smith <https://github.com/praxxis>
@@ -26,6 +26,11 @@ declare namespace meow {
         flags?: buildOptions.Options;
         autoHelp?: boolean;
         autoVersion?: boolean;
+        /**
+         * Caution: Explicitly specifying undefined for booleanDefault
+         * has different meaning from omitting key itself.
+         */
+        booleanDefault?: boolean | null;
     }
 
     interface Result {
diff --git a/types/meow/meow-tests.ts b/types/meow/meow-tests.ts
index 1b044881b5..c2059eefe6 100644
--- a/types/meow/meow-tests.ts
+++ b/types/meow/meow-tests.ts
@@ -1,6 +1,7 @@
 import meow = require('meow');
 
 const cli = meow('Help text', {
+    booleanDefault: undefined,
     flags: {
         unicorn: {
             type: 'boolean',
@@ -29,6 +30,7 @@ const cli3 = meow({
     inferType: true,
     autoHelp: true,
     autoVersion: true,
+    booleanDefault: false,
     flags: {
         unicorn: {
             type: 'boolean',