Sexual concurrency poses significant HIV/STI transmission risk. The correlates of concurrency have not been examined among homeless men. A representative sample of 305 heterosexually active homeless men utilizing meal programs in the Skid Row area of Los Angeles reported on their mental health, substance use, and social network characteristics. Nearly 40% of men reported concurrency with one of their four most recent sex partners. Results indicated that HIV seropositivity (OR = 4.39, CI: 1.10, 17.46; P = 0.04), PTSD (OR = 2.29, CI: 1.05, 5.01; P = 0.04), hard drug use (OR = 2.45, CI: 1.07, 5.58; P = 0.03), and the perception that network alters engage in risky sex (OR = 3.72, CI: 1.49, 9.30; P = 0.01) were associated with increased odds of concurrency. Programs aimed at reducing HIV/STI transmission in this vulnerable population must take into account the roles that behavioral health and social networks may play in sexual concurrency.