Objectives: Previous studies have demonstrated poor oral hygiene in children with attention deficit hyperactivity disorder (ADHD). However, the association between ADHD and periodontitis is still unclear. Methods: In all, 16,211 adolescents with ADHD and 162,110 age- and sex-matched controls participated in the study between 2001 and 2011. To identify the occurrence of periodontitis, the participants were followed up till the end of 2011. Confounding factors, including smoking, diabetes, and depressive disorder, were assessed and adjusted in the Cox regression models. Results: Adolescents with ADHD (HR: 2.29) were more likely to develop periodontitis later in life than controls. We additionally observed the beneficial effect of atomoxetine (HR: 0.42) on the periodontitis risk among adolescents with ADHD. However, this finding should be interpreted cautiously given the small sample (n = 290) of children taking atomoxetine in the present study. Conclusions: ADHD is an independent risk factor for subsequent periodontitis development. Oral health should be closely monitored in adolescents with ADHD. Future investigation of the shared pathomechanisms between periodontitis and ADHD is warranted.