Importance: Black African adults are disproportionately affected by HIV in the United Kingdom. Many within this population acquire HIV after migration or are diagnosed late. Data are needed to inform targeted interventions to increase HIV testing and prevention in this population. Objective: To inform future HIV prevention strategies by estimating diagnosed and undiagnosed HIV infection and measuring changes in HIV testing rates in black African adults in London, United Kingdom. Design, Setting, and Participants: This cross-sectional study used a self-completed survey conducted from September 20 to December 3, 2016. Questionnaires were linked to an optional, anonymous oral fluid HIV test and compared with data from a previous survey (2004). Respondents were recruited from social and commercial venues frequented by black African adults in London. Of 2531 individuals approached in 63 venues, 752 agreed to participate. Data were analyzed initially in March 2017 (as part of internal reporting) and again in August 2018. Main Outcomes and Measures: Self-reported HIV testing within the past 5 years; diagnosed and undiagnosed HIV prevalence. Logistic regression examined factors associated with HIV testing by sex. Results: In total, 292 women (median [interquartile range] age, 31 [25.0-41.5] years) and 312 men (median [interquartile range] age, 35 [25.0-41.5] years) were included in the analysis. More than half of men (159 [51.0%]) and women (154 [52.7%]) had been tested for HIV in the past 5 years. In multivariable analysis, HIV testing was associated with a range of factors in both sexes, including health service attendance, time in the United Kingdom, and sexually transmitted infection diagnosis. Increases in HIV testing in the past 5 years were observed between 2004 and 2016 for both sexes. In the 2016 sample, 219 of 235 women (93.2%) and 206 of 228 men (90.4%) tested HIV negative. Among those testing positive, 56.3% of women (9 of 16) and 40.9% of men (9 of 22) self-reported as HIV negative or untested, indicating they were living with undiagnosed HIV. A fifth of women (20.7%) and 25.0% of men reported condomless last sex with a partner of different or unknown HIV status in the past year. Conclusions and Relevance: Despite efforts to increase HIV testing, uptake in black African communities in London remains modest. This study identified a large fraction of undiagnosed infection-greater than other at-risk populations-suggesting that the prevention and care needs of this group are not adequately met.