diff --git a/lib/index.d.ts b/lib/index.d.ts
index 39a1f855..2f62be6b 100644
--- a/lib/index.d.ts
+++ b/lib/index.d.ts
@@ -27,7 +27,7 @@ declare namespace Joi {
 
     type BasicType = boolean | number | string | any[] | object | null;
 
-    type LanguageMessages = Record<string, string | Record<string, string>>;
+    type LanguageMessages = Record<string, string | Template | Record<string, string | Template>>;
 
     type PresenceMode = 'optional' | 'required' | 'forbidden';
 
@@ -650,6 +650,10 @@ declare namespace Joi {
         render?: boolean;
     }
 
+    interface ExpressionOptions extends ReferenceOptions {
+        functions?: Record<string, (...args: unknown[]) => unknown>
+    }
+
     interface StringRegexOptions {
         /**
          * optional pattern name.
@@ -2003,6 +2007,11 @@ declare namespace Joi {
         toString(): string;
     }
 
+    interface Template {
+        render(value: any, state: State, prefs: any, local: any, options?: any): string
+        toString(): string;
+    }
+
     type ExtensionBoundSchema = Schema & SchemaInternals;
 
     interface RuleArgs {
@@ -2276,7 +2285,7 @@ declare namespace Joi {
         /**
          * Generates a dynamic expression using a template string.
          */
-        expression(template: string, options?: ReferenceOptions): any;
+        expression(template: string, options?: ExpressionOptions): Template;
 
         /**
          * Creates a new Joi instance customized with the extension(s) you provide included.
@@ -2340,8 +2349,9 @@ declare namespace Joi {
 
         /**
          * Generates a dynamic expression using a template string.
+         * @see {@link Root.expression}
          */
-        x(template: string, options?: ReferenceOptions): any;
+        x(template: string, options?: ExpressionOptions): Template;
 
         // --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
         // Below are undocumented APIs. use at your own risk
diff --git a/test/index.ts b/test/index.ts
index eaac929b..48a93d11 100644
--- a/test/index.ts
+++ b/test/index.ts
@@ -69,6 +69,7 @@ validOpts = {
   messages: {
     'any.ref': str,
     'string.email': str,
+    'any.custom': Joi.x('{{.value}}')
   },
   dateFormat: 'iso',
 };
@@ -999,6 +1000,8 @@ expr = Joi.expression('{{foo}}', { iterables: true });
 expr = Joi.expression('{{foo}}', { map: [['key', 'value']] });
 expr = Joi.expression('{{foo}}', { prefix: { local: '%' } });
 expr = Joi.expression('{{foo}}', { separator: '_' });
+expr = Joi.expression('{{foo}}', { functions: { foo: (...args) => 'return' } });
+expect.type<string>({} as ReturnType<ReturnType<typeof Joi.expression>['render']>)
 
 expr = Joi.x('{{foo}}');
 expr = Joi.x('{{foo}}', { adjust: (value) => value });
@@ -1008,6 +1011,8 @@ expr = Joi.x('{{foo}}', { iterables: true });
 expr = Joi.x('{{foo}}', { map: [['key', 'value']] });
 expr = Joi.x('{{foo}}', { prefix: { local: '%' } });
 expr = Joi.x('{{foo}}', { separator: '_' });
+expr = Joi.x('{{foo}}', { functions: { foo: (...args) => 'return' } });
+expect.type<string>({} as ReturnType<ReturnType<typeof Joi.x>['render']>)
 
 // --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---