Aim To investigate pathways between unhealthy and healthy dietary patterns and periodontitis in adolescents (18–19 years of age). Materials and Methods This population‐based study (n = 2515) modelled direct and mediated pathways (via biofilm and obesity) from patterns of healthy diet (fruits, fibre, vegetables, and dairy) and unhealthy diet (sugars, snacks, and salty/fast foods) with initial periodontitis (bleeding on probing BoP, probing depth PD ≥ 4 mm, clinical attachment loss CAL ≥ 4 mm), moderate periodontitis (BoP, PD ≥ 5 mm, and CAL ≥ 5 mm), and European Federation of Periodontology and the American Academy of Periodontology (EFP‐AAP) periodontitis definitions, adjusting for sex, socio‐economic status, smoking, and alcohol, through structural equation modelling (α = 5%). Results Higher values of healthy diet were associated with lower values of initial periodontitis (standardized coefficient SC = −0.160; p < .001), moderate periodontitis (SC = −0.202; p < .001), and EFP‐AAP periodontitis (p < .05). A higher value of unhealthy diet was associated with higher values of initial periodontitis (SC = 0.134; p = .005) and moderate periodontitis (SC = 0.180; p < .001). Biofilm mediated the association between higher values of unhealthy diet and all periodontal outcomes (p < .05). Conclusions Our findings suggest that both healthy and unhealthy dietary patterns may contribute to reduced or increased extent and severity of periodontitis by local and systemic mechanisms, preceding the effect of other established causes such as smoking and obesity, in younger population.