Early puberty is associated with adverse health outcomes, but little is known regarding early life determinants influencing pubertal timing. We examined the associations between maternal gestational weight gain (GWG) and the timing of the onset of breast development (thelarche) and pubic hair development (pubarche) in a cohort of 2,070 girls born in a Kaiser Permanente Northern California facility between 2005-06. Using Weibull regression models accommodating interval censoring, and adjusting for important confounders, we found that excessive GWG was associated with increased risk of early thelarche (hazard ratio [HR]: 1.50; 95% confidence interval [CI]: 1.26-1.78) and early pubarche (HR: 1.35; 95% CI: 1.10-1.66). Inadequate GWG was associated with early thelarche (HR: 1.36; 95% CI: 1.08-1.71). The associations between excess or inadequate GWG and risk of earlier thelarche were stronger if mothers were obese before or at the beginning of pregnancy (body mass index ≥30) (HR: 2.01; 95% CI: 1.53-2.63; HR: 2.08; 95% CI: 1.45-2.98, respectively]. Similar associations were found for pubarche outcome. Inclusion of girls’ prepubertal body mass index slightly attenuated these associations, but they remained significant. Monitoring of maternal weight before and throughout pregnancy may help prevent early pubertal onset and subsequent negative health outcomes.