AbstractBackgroundTo explore the effects of a multicomponent–multilevel school‐based educational intervention on the nutritional habits and abdominal obesity indices of a representative adolescent sample.Materials and methodsA representative sample of 1610 adolescents aged 12–17 years in 23 public high schools of three municipalities in the Attica region in Greece participated in a programme funded by the European Union (August 2013–August 2014). Participants underwent dietary assessment with the use of the Mediterranean Diet Quality Index in children and adolescents (KIDMED), blood pressure (BP) assessment and screening for general and abdominal obesity by measuring body mass index, waist circumference (WC) and waist‐to‐height ratio (WHtR), at baseline and following a 6‐month school‐based intervention. The intervention involved nutritional education, physical activity and body image awareness using a multilevel approach to the adolescent participants, their parents, school teachers and health staff.ResultsAnalysis included 1032 adolescents (mean age ± SD 14·1 ± 1·6 years). Following intervention, mean KIDMED score ± SD increased significantly from 5·6 ± 2·4 to 5·8 ± 2·4 (P = 0·004). Higher percentage of boys (P = 0·028) and younger adolescents (P < 0·001) had optimal KIDMED scores ≥ 8. Significant decreases were observed in overweight and obesity (P = 0·033), mean systolic (P = 0·049) and diastolic (P < 0·001) BP, WC (P < 0·001) and WHtR (P < 0·001). WC decreased as the KIDMED score increased (P = 0·020). Living with both parents (P = 0·036), higher maternal (P = 0·039) and paternal (P = 0·004) education and having a younger father (P = 0·034) were associated with better adherence to Mediterranean diet, post‐intervention.ConclusionsIncreased adherence to MD was associated with decreased WC, indicating a potential of multicomponent–multilevel school‐based interventions to combat adolescent abdominal obesity.