One peanut butter sandwich a day (or a couple days a week) along with a balanced diet likely won't make you gain weight. As with everything, moderation is important so don't go overboard! If you eat this version of a peanut butter sandwich (paired with some fresh veggies on the side), it'll likely keep you full and satisfied until your next meal and you'll be less likely to snack on unhealthy items in between meals. Studies have shown that snacking on controlled amounts of peanut butter or peanuts is a good way to control hunger without weight gain (source), since 2 tablespoons of peanut butter has 190 calories, 17 grams of fat, and 7 grams of protein.