Objectives. To assess changes in human papillomavirus (HPV) vaccine initiation for adolescent girls and boys in Rhode Island compared with all other states. Methods. We estimated the gender-specific effects of Rhode Island's school-entry HPV vaccination policy on self-reported HPV vaccination initiation by using a difference-in-differences design with the National Immunization Survey-Teen from 2010 through 2016. Results. Compared with boys in other states, boys in Rhode Island increased their HPV vaccine initiation rate by 11% (b = 0.11; 95% confidence interval [CI] = 0.05, 0.18) after enactment of the requirement. No difference was seen in the probability of HPV vaccine initiation among girls in Rhode Island compared with girls in the multistate control (b = -0.01; 95% CI = -0.08, 0.05). Conclusions. Our analysis identified an 11% increase in HPV vaccine initiation rate among boys in Rhode Island after the school-entry requirement was enacted, whereas no significant change was observed for girls. Public Health Implications. Given suboptimal vaccine uptake rates in the United States, continued pursuit of state-level public policy to improve HPV vaccination is needed. School-entry requirements for HPV vaccination may be a strategy for closing the gap in HPV vaccine uptake for boys and girls.