Diets that are high in fat cause overeating and weight gain in multiple species of animals, suggesting that high dietary fat is sufficient to cause obesity. However, high-fat diets are typically provided freely to animals in obesity experiments, so it remains unclear whether high-fat diets would still cause obesity if these diets required more effort to obtain. We hypothesized that unrestricted access to high-fat diets is important for these diets to induce overeating and that requiring mice to perform small amounts of work to obtain a high-fat diet would reduce calorie intake and associated weight gain. To test this hypothesis, we developed a novel home-cage-based feeding device that provided the high-fat diet in two conditions: either freely or after mice poked their noses into a port one time-a simple action that is easy for them to do. Consistent with our hypothesis, requiring mice to nose-poke reduced high-fat diet intake and nearly completely prevented weight gain. Requiring mice to nose-poke also reduced low-fat grain-based pellet intake, confirming that this is a general mechanism governing food choice and not something specific to a high-fat diet. We conclude that unrestricted access to food promotes overeating and that requiring a simple action such as a nose-poke can reduce overeating and weight gain in mice. Our results may have implications for why overeating and obesity are common in modern food environments, which are often characterized by easy access to low-cost unhealthy foods.