Because early puberty has been linked to diseases later in life, identification of modifiable causes of early puberty is of interest. We explored the possible associations between maternal smoking during pregnancy and pubertal development in sons and daughters. Between 2012 and 2017, 15,819 children from the Danish National Birth Cohort, born during 2000–2003, provided half-yearly information on puberty from the age of 11 years. We estimated adjusted age differences (in months) at attaining various pubertal milestones, including Tanner stages, per 10 daily cigarettes smoked in the first trimester of gestation. In sons, exposure to smoking in utero was associated with earlier genital development (Tanner 2, −1.3 months, 95% confidence interval (CI): −2.5, 0.0; Tanner 5, −3.7 months, 95% CI: −5.3, −2.0), pubic hair development (Tanner 2, −1.8 months, 95% CI: −2.9, −0.6; Tanner 5, −2.9 months, 95% CI: −4.2, −1.7), and voice break (−2.4 months, 95% CI: −3.6, −1.3). In daughters, maternal smoking was associated with earlier breast development (Tanner 2, −3.4 months, 95% CI: −5.3, −1.5; Tanner 5, −4.7 months, 95% CI: −6.5, −2.9), pubic hair development stages 3–5 (Tanner 5, −2.5 months, 95% CI: −4.1, −1.0), and menarche (−3.1 months, 95% CI: −4.0, −2.3). Fetal exposure to tobacco smoke might advance timing of puberty in boys and girls.