Bring a large pot of salted water to a boil over medium-high heat. Add corn; cook for 3 minutes. Remove corn and rinse in cold water. When corn is cool enough to handle, use a sharp knife to slice kernels off each cob into a large bowl. Discard cobs.

Add onion, tomatoes, bell pepper, cucumber, lemon zest and juice, oil, salt, and black pepper to corn kernels in bowl; toss until well combined. Chill for at least 1 hour. Just before serving, stir in basil.