Although there is developmental research on the prevalence of offline self-disclosure in pre-adolescence and adolescence, it is still unknown (a) how boys’ and girls’online self-disclosure develops in this period and (b) how online and offline self-disclosure interact with each other. We formulated three hypotheses to explain the possible interaction between online and offline self-disclosure: the displacement, the rich-get-richer, and the rehearsal hypothesis. We surveyed 690 pre-adolescents and adolescents (10–17 years) at three time points with half-year intervals in between. We found significant gender differences in the developmental trajectories of self-disclosure. For girls, both online and offline self-disclosure increased sharply during pre- (10–11 years) and early adolescence (12–13 years), and then stabilized in middle and late adolescence. For boys, the same trajectory was found although the increase in self-disclosure started 2 years later. We found most support for the rehearsal hypothesis: Both boys and girls seemed to use online self-disclosure to rehearse offline self-disclosure skills. This particularly held for boys in early adolescence who typically have difficulty disclosing themselves offline.