This research evaluates condom-use self-efficacy and its increase throughout adolescence. Documentation of the development of condom-use self-efficacy would be important for prevention efforts given the influence of self-efficacy on actual condom usage. This study assesses a hypothesized mediation mechanism of the development of self-efficacy using a mediation analysis approach. The participants, 497 Grade-6 Bahamian students, were randomly assigned to the control condition in a 3-year longitudinal HIV prevention program trial. Condom–use self-efficacy consistently increased and condom-use self-efficacy assessed at earlier periods was positively associated with its values at subsequent periods. Additionally, self-efficacy assessed between two time points 1 year apart or longer (e.g., 6 months between baseline and 12 months) significantly mediated the impact of its levels at the previous assessment on the level at the subsequent assessment. To sustain program effect, HIV prevention programs should strive to enhance self-efficacy and provide reinforcing “boosters” no later than 12 months postintervention.