Animal-based ingredients dominate dog and cat food, but growing concerns about animal welfare and the environment are increasing interest in more sustainable options, such as plant-based and cultivated meat-based pet foods. This study evaluated the case for sustainable pet food using the effective altruism framework of scale, neglectedness, and tractability, and found strong support on all three. Globally, by 2018, at least 9% of land animals farmed each year were used to feed dogs and cats, with more consumed by average dogs (13) than by average people (9) annually. Switching to nutritionally sound vegan pet diets worldwide could spare around seven billion land animals and many billions of marine animals from slaughter, while freeing enough food energy to feed 519 million people. Transitioning only dogs could cut greenhouse gas emissions equal to 1.5 times the UK’s annual output and free land larger than Mexico. Despite these benefits, sustainable pet food receives little funding, attention, or talent. However, the issue is tractable: 13–18% of pet guardians would consider vegan diets if their concerns were met. Assuming only one dog or cat per guardian this could allow at least 70 million dogs and 86 million cats to transition globally, with the true figures probably several times greater. Overall, sustainable pet diets are a largely overlooked but high-impact opportunity to reduce farmed animal consumption, lower environmental harm, and improve food security.