Understanding the uptake and patterns of sexual partnerships of adolescent males reveals their risky behaviors that could persist into adulthood. Using venue-based sampling, we surveyed 671 male youth ages 15–19 from an urban Tanzanian neighborhood about their sexual partnerships during the past six months. The proportion of males who had ever had sex increased with age (21% at age 15; 70% at age 17; 94% at age 19), as did the proportion who engaged in concurrency (5% at age 15; 28% at age 17; 44% at age 19). Attendance at ≥2 social venues per day and meeting a sexual partner at a venue was associated with concurrency. Concurrency was associated with alcohol consumption before sex among 18–19 year olds and with not being in school among 15–17 year olds. We find that concurrency becomes normative over male adolescence. Venue-based sampling may reach youth vulnerable to developing risky sexual partnership patterns.