Due to the lack of studies comparing the determinants of well-being in omnivores and vegetarians, we examined associations of socio-demographic and lifestyle factors, including adherence to a Mediterranean-style diet, in relation to well-being in omnivorous, vegetarian, and vegan women. Well-being was assessed using a validated WHO-5 Well-Being Index. Adherence to the Mediterranean-style diet was determined using a modified Mediterranean diet score. The study was conducted on 636 women (23.9 ± 5.7 years), of whom 47.3% were omnivores, 33.2% vegetarians, and 19.5% vegans. The good well-being group (WHO-5 Index ≥ 13 points) comprised 30.9% of the omnivores, 46.0% of the vegetarians, and 57.3% of the vegans. The remaining participants were classified as belonging to the poor well-being group (<13 points). Compared to the omnivores, the vegetarians and vegans had a 1.6-fold (95% CI: 1.04-2.42) and a 2.4-fold (95% CI: 1.45-3.99) higher probability of having good well-being, respectively. In omnivores, the predictors of good well-being were adherence to the Mediterranean-style diet (a 1-score increment was associated with a 17% higher probability of good well-being, P-trend = 0.016), higher self-perceived health status, and lower levels of stress. In vegetarians and vegans, it was older age, higher physical activity (≥3 h/week), 7-8 h sleep time, and similarly to omnivores' higher self-perceived health status and lower stress level. Our findings indicate that following a Mediterranean-style diet was associated with better well-being in omnivores. Furthermore, we identified that different determinants were associated with well-being in omnivorous and vegetarian and vegan women.