In a large skillet over medium heat, brown the Italian sausage until the inside is no longer pink. Cut sausage into 1/4 inch slices, and continue cooking until browned.
Stir in the zucchini, yellow squash, and onion; cook and stir for 2 minutes. Pour in the tomatoes, with liquid. Reduce heat, cover, and simmer for 10 to 15 minutes.