The aim of this research was to examine the assumptions of the theory of planned behavior in the context of choosing to eat a predominantly plant-based diet. Since, according to this theory, the intention is the best predictor of actual behavior, for practical reasons, it is the intention to eat a predominantly plant-based diet that has been taken as the central construct, or criterion of this research. The aim was also to investigate whether the introduction of moral norms to the model, as a new predictor, contributes to explaining the variance of the intention to eat a predominantly plant-based diet. By conducting hierarchical regression analysis in two steps, it was established that attitudes, subjective norms, and perceived behavioral control are significant independent predictors of intention, and together they explain 69% of the variance of intention. By adding moral norms to the model, an additional 10% of the variance of the intention was explained, and moral norms, along with attitudes, proved to be the best independent predictor. T-tests confirmed gender differences in attitudes, moral norms, and intentions to eat a predominantly plant-based diet, suggesting that women generally show a more positive attitude, more pronounced internal normative pressure, and a stronger intention to choose a predominantly plant-based diet.