Preexposure prophylaxis (PrEP) is underused in persons who use drugs and justice-involved persons. In an ongoing randomized controlled trial in 4 US locations comparing patient navigation versus mobile health unit on time to initiation of HIV medication or PrEP for justice-involved persons who use stimulants or opioids and who are at risk for or living with HIV, we assessed HIV risk factors, perceived HIV risk, and interest in PrEP. Participants without HIV (n = 195) were 77% men, 65% White, 23% Black, and 26% Hispanic; 73% reported a recent history of condomless sex, mainly with partners of unknown HIV status. Of 34% (67/195) reporting injection drug use, 43% reported sharing equipment. Despite risk factors, many persons reported their risk for acquiring HIV as low (47%) or no (43%) risk, although 51/93 (55%) with PrEP indications reported interest in PrEP. Justice-involved persons who use drugs underestimated their HIV risk and might benefit from increased PrEP education efforts.