Background: Although the Mediterranean diet (MD) is recognized for its protective effect against cardiometabolic risk, adherence to this diet tends to decrease among adolescents. The KIDMED index used to assess adherence to MD in children and adolescents has not undergone a formal revision since its initial publication in 2004. Objective: To assess adherence to the MD using both the original (2004) and updated (2019) versions of KIDMED, in the same sample of adolescents. Material and Methods: A cross-sectional study was conducted with 129 adolescents. Anthropometric measurements included body weight and height to calculate the body mass index (BMI) and BMI-z-score, and waist circumference (WC) were taken, and physical activity levels were assessed using the short version of the IPAQ questionnaire. Adherence to the MD was assessed using the original version (OV, 2004) and updated versions (UV, 2019) of KIDMED. Results: Adherence to MD was generally moderate, with a mean score of 5.88 ± 3.22 according to the (OV) and 5.61 ± 3.05 according to the (UV). The latter led to an increase in the proportion of adolescents with low adherence (30.2% vs. 22.5%) and a decrease in those classified as having good adherence (25.6% vs. 28.7%). Agreement between the two classifications was moderate. Significant differences were observed for several items, including fruit consumption, breakfast quality, frequenting fast-food restaurants, and consumption of sugary products. Conclusions: The UV of KIDMED in light of current nutritional recommendations, leads to a more demanding assessment of eating behaviors and better identification of at-risk dietary patterns in adolescents, without altering the OV structure.