Watermelon and Tomato Feta Salad

1 (4 pound) chilled seedless watermelon, peeled and cut into 1-inch cubes

3 heirloom tomatoes (preferably green or orange), seeded and cut into 1-inch cubes

1 sweet onion, thinly sliced

3 tablespoons chopped fresh mint

1 cup crumbled feta cheese

1 teaspoon salt

Gently toss the watermelon, tomatoes, onion, and mint together in a large bowl; add the feta cheese, season with the salt, and gently toss again to combine.