Background: The reasons behind low adherence to the Mediterranean diet (Med-diet) are still not entirely known. We aimed to evaluate the effect of biological (i.e., sex-related) and psycho-socio-cultural (i.e., gender-related) factors on Med-diet adherence. Methods: Baseline Med-diet adherence was measured using a self-administered questionnaire among adults with ischemic heart disease (IHD) from the EVA (Endocrine Vascular Disease Approach) study. A multivariable analysis was performed to estimate the effect of sex- and gender-related factors (i.e., identity, roles, relations, and institutionalized gender) on low adherence. Results: Among 366 participants (66 ± 11 years, 31% women), 81 (22%) adults with low adherence demonstrated higher rates of diabetes, no smoking habit, lower male BSRI (Bem Sex Role Inventory) (median (IQR) 4.8 (4.1 to 5.5) vs. 5.1 (4.5 to 5.6) and p = 0.048), and higher Perceived Stress Scale 10 items (PSS-10) (median (IQR) 19 (11 to 23) vs. 15 (11 to 20) and p = 0.07) scores than those with medium-high adherence. In the multivariable analysis, only active smoking (odds ratio, OR = 2.10, 95% confidence interval, CI 1.14 to 3.85 and p = 0.017), PPS-10 (OR = 1.04, 95% CI 1.00 to 1.08, and p = 0.038) and male BSRI scores (OR = 0.70, 95% CI 0.52 to 0.95, and p = 0.021) were independently associated with low adherence. Conclusions: Male personality traits and perceived stress (i.e., gender identity) were associated with low Med-diet adherence regardless of the sex, age, and comorbidities. Therefore, gender-sensitive interventions should be explored to improve adherence in IHD.