At a time when the health benefits of the Mediterranean diet (MD) are pervasively recognised, a paradoxical observation is the decreasing adherence to this dietary pattern in its native countries. This study aims to investigate temporal trends in adherence to the MD among adolescents (10–19 years old) in Lebanon. Data were drawn from three national cross-sectional surveys conducted at three points in time: 1997 (n 2004), 2009 (n 3656) and 2015 (n 1204). Dietary intake was assessed using 24-h dietary recalls, and adherence to the MD was assessed using two country-specific indexes: the composite Mediterranean diet (c-MED) index and Lebanese Mediterranean diet (LMD) index. Significant decreases in c-MED and LMD scores and in the proportion of adolescents adhering to the MD were observed between 1997 and 2015, with more consistent results among females (P < 0·05). Projections for the year 2030 showed further decreases, with less than a quarter of adolescents remaining adherent to the MD. Based on linear regression analyses, belonging to the year 2009 was associated with significantly lower MD scores compared with 1997, even after adjustment for potential covariates (c-MED β = –0·16, 95 % CI –0·30, –0·01; LMD β = –0·42, 95 % CI –0·67, –0·17). Similar results were obtained when comparing survey year 2015 with 1997 (c-MED score β = –0·20, 95 % CI –0·33, –0·06; LMD score β = –0·60, 95 % CI –0·82, –0·37). Findings highlight the erosion of the MD among Lebanese adolescents and underline the need for a comprehensive food system approach that fosters the promotion of the MD as a nutritionally balanced and sustainable dietary pattern.