diff --git a/lib/rules/no-array-index-key.js b/lib/rules/no-array-index-key.js
index ceb2a1676a..d75988a833 100644
--- a/lib/rules/no-array-index-key.js
+++ b/lib/rules/no-array-index-key.js
@@ -150,6 +150,40 @@ module.exports = {
         identifiers.filter(isArrayIndex).forEach(() => {
           context.report({node, message: ERROR_MESSAGE});
         });
+
+        return;
+      }
+
+      if (node.type === 'CallExpression'
+          && node.callee
+          && node.callee.type === 'MemberExpression'
+          && node.callee.object
+          && isArrayIndex(node.callee.object)
+          && node.callee.property
+          && node.callee.property.type === 'Identifier'
+          && node.callee.property.name === 'toString'
+      ) {
+        // key={bar.toString()}
+        context.report({
+          node,
+          message: ERROR_MESSAGE
+        });
+        return;
+      }
+
+      if (node.type === 'CallExpression'
+          && node.callee
+          && node.callee.type === 'Identifier'
+          && node.callee.name === 'String'
+          && Array.isArray(node.arguments)
+          && node.arguments.length > 0
+          && isArrayIndex(node.arguments[0])
+      ) {
+        // key={String(bar)}
+        context.report({
+          node: node.arguments[0],
+          message: ERROR_MESSAGE
+        });
       }
     }
 
diff --git a/tests/lib/rules/no-array-index-key.js b/tests/lib/rules/no-array-index-key.js
index ec4370298c..7f8b691e41 100644
--- a/tests/lib/rules/no-array-index-key.js
+++ b/tests/lib/rules/no-array-index-key.js
@@ -75,6 +75,22 @@ ruleTester.run('no-array-index-key', rule, {
       code: 'foo.map((baz, i) => <Foo key />)'
     },
 
+    {
+      code: 'foo.map((bar, i) => <Foo key={i.baz.toString()} />)'
+    },
+
+    {
+      code: 'foo.map((bar, i) => <Foo key={i.toString} />)'
+    },
+
+    {
+      code: 'foo.map((bar, i) => <Foo key={String()} />)'
+    },
+
+    {
+      code: 'foo.map((bar, i) => <Foo key={String(baz)} />)'
+    },
+
     {
       code: 'foo.reduce((a, b) => a.concat(<Foo key={b.id} />), [])'
     },
@@ -119,6 +135,16 @@ ruleTester.run('no-array-index-key', rule, {
       errors: [{message: 'Do not use Array index in keys'}]
     },
 
+    {
+      code: 'foo.map((bar, i) => <Foo key={i.toString()} />)',
+      errors: [{message: 'Do not use Array index in keys'}]
+    },
+
+    {
+      code: 'foo.map((bar, i) => <Foo key={String(i)} />)',
+      errors: [{message: 'Do not use Array index in keys'}]
+    },
+
     {
       code: 'foo.map((bar, anything) => <Foo key={anything} />)',
       errors: [{message: 'Do not use Array index in keys'}]