Non-time-consuming and easy-to-administer dietary assessment tools specific for pregnancy are needed. The aim of this validation study nested in the IMPACT BCN (Improving Mothers for a better PrenAtal Care Trial BarCeloNa) trial is to determine the concurrent validity of the 17-item pregnancy-adapted Mediterranean diet score (preg-MEDAS) and to analyze whether changes in the preg-MEDAS score were associated with maternal favorable dietary and cardiometabolic changes after 3 mo of intervention in pregnant women. Dietary data was collected in 812 participants using the preg-MEDAS and a 151-item validated food frequency questionnaire (FFQ) at baseline (19–23 wk gestation) and final visit (31–34 wk gestation). Concurrent preg-MEDAS validity was evaluated by Pearson and intraclass correlation coefficients, κ statistic, and Bland-Altman methods. The preg-MEDAS had a good correlation with the FFQ (r = 0.76 and intraclass correlation coefficient 0.75). The agreement of each of the preg-MEDAS items ranged from 40.9% to 93.8% with a substantial agreement mean concordance (κ = 0.61). A 2-point increase in preg-MEDAS was associated with a decrease in maternal mean and systolic blood pressure (β: −0.51 mmHg; 95% confidence interval CI: −0.97, −0.04 mmHg and −0.87 mmHg; 95% CI: −1.48, −0.26 mmHg, respectively). The preg-MEDAS displays good validity for assessing adherence to the Mediterranean diet, allowing detection of dietary changes over time. In addition, changes observed in preg-MEDAS are significantly associated with a decrease in maternal blood pressure. Therefore, we propose preg-MEDAS as a rapid and simple dietary assessment tool during pregnancy. This trial was registered at clinicaltrials.gov as NCT03166332.