diff --git a/packages/@aws-cdk/aws-iam/lib/policy-statement.ts b/packages/@aws-cdk/aws-iam/lib/policy-statement.ts
index 2c07104874c1a..425a69e6b8a4d 100644
--- a/packages/@aws-cdk/aws-iam/lib/policy-statement.ts
+++ b/packages/@aws-cdk/aws-iam/lib/policy-statement.ts
@@ -167,7 +167,8 @@ export class PolicyStatement {
    * Add a condition to the Policy
    */
   public addCondition(key: string, value: any) {
-    this.condition[key] = value;
+    const existingValue = this.condition[key];
+    this.condition[key] = existingValue ? { ...existingValue, ...value } : value;
   }
 
   /**
diff --git a/packages/@aws-cdk/aws-iam/test/policy-document.test.ts b/packages/@aws-cdk/aws-iam/test/policy-document.test.ts
index 3f5388e6d75de..1b4e359e98d4b 100644
--- a/packages/@aws-cdk/aws-iam/test/policy-document.test.ts
+++ b/packages/@aws-cdk/aws-iam/test/policy-document.test.ts
@@ -572,4 +572,19 @@ describe('IAM polocy document', () => {
 
     expect(stack.resolve(doc1)).toEqual(stack.resolve(doc2));
   });
+
+  test('adding another condition with the same operator does not delete the original', () => {
+    const stack = new Stack();
+
+    const p = new PolicyStatement();
+
+    p.addCondition('StringEquals', { 'kms:ViaService': 'service' });
+
+    p.addAccountCondition('12221121221');
+
+    expect(stack.resolve(p.toStatementJson())).toEqual({
+      Effect: 'Allow',
+      Condition: { StringEquals: { 'kms:ViaService': 'service', 'sts:ExternalId': '12221121221' } }
+    });
+  });
 });