This study aims to identify the relationships between breakfast habits, leisure-time sedentary behavior, and suicidal behaviors among Korean adolescents, including the mediating effect of leisure-time sedentary behavior on the relationship between breakfast habits and suicidal behaviors. We conducted a cross-sectional national study using secondary data from the 2017–2019 (13–15th) Korea Youth Risk Behavior Web-Based Surveys, analyzing data from a final sample of 153,992 Korean adolescents using multivariate logistic regression. No breakfast habits were statistically significantly related to suicidal ideation (crude OR [COR], 95% CI = 1.218, 1.172–1.265), suicidal plans (COR, 95% CI = 1.305, 1.228–1.385), and suicide attempts (COR, 95% CI = 1.533, 1.432–1.642). The effects of breakfast habits (independent variable) on suicidal behaviors (outcome variables) were mediated by leisure-time sedentary behavior (mediating variable). Leisure-time sedentary behavior had a statistically significant indirect effect on breakfast habits and suicidal behaviors (p < 0.05). The mediating effect size of breakfast habits mediated by leisure-time sedentary behavior was 3.46% for suicidal ideation, 2.48% for suicidal plans, and 1.06% for suicide attempts. Adolescents who did not consume breakfast demonstrated a significantly higher possibility of suicidal ideation, suicidal plans, and suicide attempts. Parents and teachers should be aware of and monitor adolescents’ leisure-time sedentary behavior and breakfast habits to prevent suicidal behavior among this age group.