Abstract Context Pubertal timing in boys is associated with body mass index (BMI). Studies consistently report an inverse correlation of BMI and pubertal timing within the normal BMI range. However, observations in obese boys are conflicting with different studies reporting either early or delayed pubertal onset in obese boys. Objective We aimed to assess the association of male pubertal timing with age-specific BMI (zBMI) in obese boys. Design, Setting, and Participants A total of 218 obese boys (zBMI > +2SD, with a median age at baseline of 10.8 years (range 4.2–17.0), were recruited as part of a prospective outpatient childhood obesity intervention program at Nordsjællands Hospital, Hillerød, Denmark, between 2009 and 2017. Serving as controls, we included 660 healthy boys participating in the population-based COPENHAGEN Puberty Study (-2SD < zBMI ≤ +2SD, 2006–2014). Subanalyses were performed on overweight controls (+1SD < zBMI ≤ +2SD). The clinical assessment of pubertal development by Tanner staging, including testis volume using a Prader’s orchidometer, was performed by trained physicians. The timing of pubertal milestones was estimated by probit analyses. Main Outcome Measures Timing of testicular volume ≥ 4 mL, genital stage ≥ 2, and pubarche. Results The mean (95% confidence interval [CI]) age of onset of pubertal event in obese boys was as follows: testicular volume ≥ 4 mL, 11.3 years (11.0–11.6); genital stage ≥ 2, 11.6 yrs (11.3–11.9); and pubarche, 11.9 years (11.5–12.3). Testicular volume ≥ 4 mL occurred significantly earlier in obese boys compared to controls (-2SD < zBMI ≤ +2SD) (P = 0.01). We did not observe significant differences for either the timing of pubarche nor the genital stage ≥ 2 (P = 0.06 and P = 0.94, respectively) Conclusions We demonstrate that testicular enlargement in obese boys occurs significantly earlier compared to a population-based normal-weight reference cohort.