diff --git a/types/helmet/helmet-tests.ts b/types/helmet/helmet-tests.ts
index 8b4c5634ac..39414a7d5f 100644
--- a/types/helmet/helmet-tests.ts
+++ b/types/helmet/helmet-tests.ts
@@ -119,6 +119,13 @@ function hpkpTest() {
         sha256s: ['AbCdEf123=', 'ZyXwVu456='],
     }));
 
+    app.use(helmet.hpkp({
+        maxAge: 7776000000,
+        sha256s: ['AbCdEf123=', 'ZyXwVu456='],
+        includeSubDomains: false
+    }));
+
+    // Deprecated: Use includeSubDomains instead. (Uppercase "D")
     app.use(helmet.hpkp({
         maxAge: 7776000000,
         sha256s: ['AbCdEf123=', 'ZyXwVu456='],
@@ -128,7 +135,7 @@ function hpkpTest() {
     app.use(helmet.hpkp({
         maxAge: 7776000000,
         sha256s: ['AbCdEf123=', 'ZyXwVu456='],
-        includeSubdomains: true
+        includeSubDomains: true
     }));
 
     app.use(helmet.hpkp({
@@ -162,6 +169,12 @@ function hstsTest() {
       maxAge: 7776000000,
     }));
 
+    app.use(helmet.hsts({
+      maxAge: 7776000000,
+      includeSubDomains: true
+    }));
+
+    // Deprecated: Use includeSubDomains instead. (Uppercase "D")
     app.use(helmet.hsts({
       maxAge: 7776000000,
       includeSubdomains: true
diff --git a/types/helmet/index.d.ts b/types/helmet/index.d.ts
index b0a50c901e..1a8c713da1 100644
--- a/types/helmet/index.d.ts
+++ b/types/helmet/index.d.ts
@@ -1,6 +1,6 @@
 // Type definitions for helmet
 // Project: https://github.com/helmetjs/helmet
-// Definitions by: Cyril Schumacher <https://github.com/cyrilschumacher>, Evan Hahn <https://github.com/EvanHahn>, Elliot Blackburn <https://github.com/bluehatbrit>
+// Definitions by: Cyril Schumacher <https://github.com/cyrilschumacher>, Evan Hahn <https://github.com/EvanHahn>, Elliot Blackburn <https://github.com/bluehatbrit>, Daniel Müller <https://github.com/chdanielmueller>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -134,7 +134,11 @@ declare namespace helmet {
     export interface IHelmetHpkpConfiguration {
         maxAge: number;
         sha256s: string[];
+        /**
+         * @deprecated Use includeSubDomains instead. (Uppercase "D")
+         */
         includeSubdomains?: boolean;
+        includeSubDomains?: boolean;
         reportUri?: string;
         reportOnly?: boolean;
         setIf?: IHelmetSetIfFunction;
@@ -142,7 +146,11 @@ declare namespace helmet {
 
     export interface IHelmetHstsConfiguration {
         maxAge?: number;
+        /**
+         * @deprecated Use includeSubDomains instead. (Uppercase "D")
+         */
         includeSubdomains?: boolean;
+        includeSubDomains?: boolean;
         preload?: boolean;
         setIf?: IHelmetSetIfFunction;
         force?: boolean;
diff --git a/types/koa-helmet/koa-helmet-tests.ts b/types/koa-helmet/koa-helmet-tests.ts
index 2290ab07fa..34bca1958a 100644
--- a/types/koa-helmet/koa-helmet-tests.ts
+++ b/types/koa-helmet/koa-helmet-tests.ts
@@ -95,13 +95,13 @@ function hpkpTest() {
     app.use(helmet.hpkp({
         maxAge: 7776000000,
         sha256s: ['AbCdEf123=', 'ZyXwVu456='],
-        includeSubdomains: false
+        includeSubDomains: false
     }));
 
     app.use(helmet.hpkp({
         maxAge: 7776000000,
         sha256s: ['AbCdEf123=', 'ZyXwVu456='],
-        includeSubdomains: true
+        includeSubDomains: true
     }));
 
     app.use(helmet.hpkp({
@@ -137,7 +137,7 @@ function hstsTest() {
 
     app.use(helmet.hsts({
       maxAge: 7776000000,
-      includeSubdomains: true
+      includeSubDomains: true
     }));
 
     app.use(helmet.hsts({