Background: Dietary behaviors and lifestyle factors co-occur in integrated profiles that shape cardiometabolic and mental health risk. Sustainable nutrition, particularly plant-forward eating, has become a relevant dimension of dietary quality, yet its adoption varies across population subgroups and may differ by sex. This study examined sex divergences in sustainable nutrition and lifestyle profiles among Italian adults and the potential indirect association of sustainability awareness in the relationship between sex and plant-based diet adherence. Methods: A cross-sectional study was conducted with 1066 Italian adults using the validated Nutritional and Social Health Habits inventory for Italian people (NutSo-HH-Ita) to assess sustainable nutrition and lifestyle factors. Data were analyzed using independent-samples t-tests and Chi-square tests for sex comparisons, mediation analysis to examine the indirect effect of sustainability awareness on the sex–plant-based diet association, and K-means clustering with multinomial logistic regression to identify lifestyle phenotypes and test sex as a predictor of cluster membership, while adjusting for age, education, and income. Results: Women showed higher plant-based diet adherence (Cohen’s d = 0.33; 95% confidence intervals (CI) [0.20, 0.46]) and higher sustainability awareness (Cohen’s d = 0.14), while men reported higher physical activity (Cohen’s d = 0.21). Sustainability awareness was statistically associated with a small indirect component in the association between sex and plant-based diet adherence (p = 0.045). Cluster analysis identified distinct lifestyle phenotypes, and sex independently predicted phenotype membership after covariate adjustment (p < 0.001), with men more likely to belong to a profile characterized by relatively higher activity combined with poorer sustainability engagement and less healthy lifestyle habits. Conclusions: Sex differences in sustainable nutrition are embedded within broader lifestyle phenotypes. Integrating sustainability-related predispositions with behavioral phenotyping may support sex-sensitive prevention strategies and help identify “Active-Low Engagement” male subgroup that could be overlooked when evaluating single behaviors in isolation.