Partner notification and treatment are essential components of sexually transmitted infection (STI) management, but little is known about such practices among adolescents and young adults. Using data from a prospective cohort study (AYAZAZI) of youth aged 16–24 years in Durban, South Africa, we assessed the STI care cascade across participant diagnosis, STI treatment, partner notification, and partner treatment; index recurrent STI and associated factors; and reasons for not notifying partner of STI. Participants completed laboratory-based STI screening (Chlamydia trachomatis, Neisseria gonorrhoeae, Mycoplasma genitalium, Trichomonas vaginalis) at enrollment and at 12 months. Of the 37/216 participants with STI (17%), 27/37 (73%) were women and 10/37 (27%) were men. Median age was 19 years (IQR: 18–20). Of the participants with STI, 23/37 (62%) completed a Treatment and Partner Tracing Survey within 6 months of diagnosis. All survey participants reported completing STI treatment (100%), 17/23 (74%) notified a partner, and 6/23 (35%) reported partner treatment. Overall, 4/23 (11%) participants had 12-month recurrent C. trachomatis infection, with no association with partner notification or treatment. Stigma and lack of STI knowledge were reasons for not notifying partner of STI. STI partner notification and treatment is a challenge among youth. Novel strategies are needed to overcome barriers along the STI care cascade.