Based on laboratory analyses of both vegan and meat-based pet foods, some studies have suggested pet food samples of either type to be nutritionally sound, while others have demonstrated nutritional flaws. However, a more systematic evaluation of the nutritional soundness of vegan and meat-based pet foods has been lacking. Consequently, this study sought to more comprehensively review relevant published studies to date. Overall, 54 suitable studies were located and assessed. In total, 81.5% of the studies found nutritional inadequacies in the pet foods they examined. These nutritional inadequacies spanned multiple pet food types—both vegan and meat-based, and both dog and cat food. The nutritional inadequacies concerned multiple nutrients. When considered overall, vegan diets were not more likely than meat-based diets to be nutritionally unsound. More testing of all pet food types is required to ensure these fulfil the nutritional needs of dogs and cats.