IntroductionAdolescent girls and young women (AGYW) disproportionately experience gender-based violence (GBV), which can increase during emergencies like the COVID-19 pandemic.MethodsA cohort of youth ages 15–24 in Nairobi, Kenya was surveyed at three time points over an 18-month period prior to and during the COVID-19 pandemic: June–August 2019 (prepandemic), August–October 2020 (12-month follow-up) and May 2021 (18-month follow-up). We characterise (1) prevalence, relative timing and help-seeking for leading forms of GBV, (2) GBV trajectories over 18 months and (3) associations of individual, dyad and COVID-related factors on GBV trajectories among AGYW (n=612) in Nairobi, Kenya. Virtual focus group discussions (n=12) and interviews (n=40) contextualise quantitative results.ResultsIntimate partner violence (IPV) prevalence hovered at 17% across time points (ever at pre-pandemic; past 12 months at 12-month follow-up (2020); past 6 months at 18-month follow-up (2021)); non-partner sexual violence (SV) was 3% at 12-month and 18-month follow-up. Overall, 27.6% of AGYW experienced IPV during the pandemic. IPV during the pandemic was associated with work as the primary pre-COVID activity, low social support and partner age difference >4 years. Among AGYW partnered at all three time points, 66.2% stayed IPV-free (no IPV), 9.2% saw IPV resolve by 18-month follow-up, while 11.1% had IPV start and 13.6% experienced intermittent IPV. Help-seeking for IPV and SV in 2020 (11.1% and 4.6%, respectively) increased to 21.7% and 15.1%, respectively, by 2021. Qualitative results speak to impacts of curfews, and pandemic-related financial stress in prompting conflict and threatening traditional gender roles, and underlying conditions that enable IPV.ConclusionThe persistence of IPV against AGYW in Nairobi prior to and during the COVID-19 pandemic reflects endemic conditions and pandemic-specific stressors. Youth, including unmarried youth, remain a priority population for GBV prevention and survivor-centred response.