Although the expected benefits of the Mediterranean diet (MD) are comprehensive, its implementation is hampered by poor adherence. Several factors can affect adherence to MD guidelines. The current study aimed to explore sociodemographic and pregnancy-related determinants of MD adherence among Saudi women. A correlational cross-sectional research design was conducted on a snowball sample of 774 pregnant women from the Najran region, Saudi Arabia, using an online survey between February and May 2024. A self-administered questionnaire consisting of sociodemographic data, pregnancy-related characteristics, and the MD scale was used for data collection. The current study showed that only 32.2% of participants had high adherence to the MD, and 57.6% had moderate adherence. Regarding sociodemographic determinants of MD adherence, highly educated, older women with lower pre-pregnancy body mass index (BMI) and higher monthly income increased the probability of high adherence to the MD (p < 0.05). In addition, being physically active before or during pregnancy significantly increased the woman’s probability of having higher adherence to the MD (p < 0.05). Concerning pregnancy-related determinants, having a planned pregnancy and regular antenatal care (ANC) increased the woman’s probability of high adherence to the MD by nearly 1.3 times (p < 0.05). In addition, low adherence to the MD increases the risk of gestational diabetes. In conclusion, numerous sociodemographic and pregnancy-related determinacies can significantly affect a woman’s adherence to the MD. Healthcare providers should address these determinants during the planning and implementation of pregnant women’s nutritional counseling to make the counseling process woman-centered and more effective.