We investigated the influence of partner-provided HIV-specific and general social support on the sexual risk behavior of gay male couples with concordant, discordant, or serostatus-unknown outside partners. Participants were 566 gay male couples from the San Francisco Bay Area. HIV-specific social support was a consistent predictor for reduced unprotected anal intercourse (UAI) with both concordant outside partners (all couple types) and outside partners of discordant or unknown serostatus (concordant negative and discordant couples). General social support was associated with increased UAI with concordant outside partners for concordant negative and concordant positive couples (i.e., serosorting). Our findings suggest that prevention efforts should target couples and identify the level of HIV-specific support that partners provide. Partner-provided support for HIV-related behaviors could be an additional construct to consider in gay male relationships, akin to relationship satisfaction and commitment, as well as an important component of future HIV prevention interventions.