Background: The primary goal of this study was to investigate the relation between the choice of a vegan or vegetarian diet as a criterion of sustainability and the aspect of heartfulness. We also analyzed which demographic, diet-related, and mindfulness practice-related variables could predict the different facets of heartfulness. Methods: In total, 419 persons participated. After providing demographic, diet-related, and mindfulness practice-related information, participants completed a gratitude questionnaire, a self-compassion scale, a compassion scale, and an equanimity scale. Results: The results show that vegans and vegetarians indicated higher scores than omnivores in some aspects of heartfulness, such as both self-compassion scales. These effects could not be shown for the two equanimity scales and for the gratitude questionnaire. Most aspects of heartfulness could either be predicted by demographic or diet-related variables. The best predictors of the elements of heartfulness were the ecological, ethical, or health-related reasons for choosing their diet stated by the participants, as well as the importance the participants attached to nutrition. Conclusion: This study provides evidence that vegans and vegetarians scored higher in several aspects of heartfulness. Vegans tended to score even higher than vegetarians. Both demographic and diet-related variables could predict heartfulness.