HIV-1 RNA genetic diversity predicts time since infection, which is important for clinical care and research. It is unclear, however, whether proviral DNA genetic diversity sampled under suppressive antiretroviral therapy can be used for this purpose. We tested whether proviral genetic diversity from next-generation sequencing predicts time since infection and recency in 221 people with HIV-1 with known infection time. Proviral diversity was significantly associated with time since infection (P < 5×10-7, R2 up to 25%) and predictive of treatment initiation during recent infection (area under the curve-receiver operating characteristic up to 0.85). This shows the utility of proviral genetic diversity as a proxy for time since infection.