diff --git a/types/knockout/index.d.ts b/types/knockout/index.d.ts
index 5ac6943eb4..1340e22d8f 100644
--- a/types/knockout/index.d.ts
+++ b/types/knockout/index.d.ts
@@ -17,7 +17,7 @@ interface KnockoutComputedFunctions<T> {
 }
 
 interface KnockoutObservableFunctions<T> {
-    equalityComparer(a: any, b: any): boolean;
+    equalityComparer(a: T, b: T): boolean;
 }
 
 interface KnockoutObservableArrayFunctions<T> {
@@ -78,6 +78,10 @@ interface KnockoutComputedStatic {
 interface KnockoutComputed<T> extends KnockoutObservable<T>, KnockoutComputedFunctions<T> {
     fn: KnockoutComputedFunctions<any>;
 
+    // It's possible for a to be undefined, since the equalityComparer is run on the initial
+    // computation with undefined as the first argument. This is user-relevant for deferred computeds.
+    equalityComparer(a: T | undefined, b: T): boolean;
+
     dispose(): void;
     isActive(): boolean;
     getDependenciesCount(): number;
diff --git a/types/knockout/test/index.ts b/types/knockout/test/index.ts
index 38f439efb7..130527818e 100644
--- a/types/knockout/test/index.ts
+++ b/types/knockout/test/index.ts
@@ -601,6 +601,13 @@ function test_misc() {
 		}
 	}
 
+    ko.observable("foo").equalityComparer = (a, b) => {
+        return a.toLowerCase() === b.toLowerCase();
+    };
+    ko.computed(() => "foo").equalityComparer = (a, b) => {
+        return (a !== undefined) && a.toLowerCase() === b.toLowerCase();
+    };
+
 }
 
 interface KnockoutBindingHandlers {