Bring a large pot of salted water to a boil. Add spaghetti and cook according to package instructions until al dente.
Stir in the drained tuna, capers, and lemon juice. Cook until the tuna is heated through, about 2 minutes.
Add the cooked spaghetti to the skillet with 1/2 cup pasta water. Toss well to coat the pasta with the tuna mixture.
Season with salt and pepper to taste and adjust acidity with more lemon juice if needed. Stir in chopped parsley.