Chronic heart failure (CHF) is a major health problem, representing the main cause of hospitalization in people over 65 years of age. Several studies have associated the Mediterranean diet with a cardioprotective function, improving prognoses in patients with high cardiovascular risk. Our main objective is to determine whether higher adherence to the Mediterranean diet is associated with a lower severity of CHF, based on the number of decompensations and disease complications. This study was a single-center retrospective cohort study conducted at the Virgen del Rocío Hospital (Seville). Adherence to a Mediterranean diet was determined by the Mediterranean Diet Adherence Screener (MEDAS) in patients with chronic heart failure in a state of clinical stability, the number of decompensations in the 12 months before inclusion, cardiac biomarkers (NT-proBNP and CA125), evaluation of dyspnea, and quality of life assessment according to NYHA and KCCQ scales and analytical profiles. Seventy-two patients were included (35 with high adherence to the Mediterranean diet and 37 with low adherence). The mean age was 81.29 ± 0.86 years. A trend towards fewer decompensations (1.49 ± 0.14 vs. 1.92 ± 0.17, p = 0.054) and lower NT-proBNP values (2897.02 ± 617.16 vs. 5227.96 ± 1047.12; p = 0.088) was observed in patients with high adherence compared to those with low adherence to the Mediterranean diet. Our results suggest that patients with CHF and high adherence to the Mediterranean diet have a tendency towards an improved cardiac profile, indicated by fewer decompensations and lower NT-proBNP levels. Future clinical trials are needed to substantiate these hypotheses.