Public stigma surrounding HIV is related to heightened emotional distress, poor psychological functioning, and reduced subjective well-being in people living with HIV. For men who have sex with men (MSM) living with HIV, they may also face stigmatizing attitudes within the gay community, which create an additional burden to their health. Grounded in the psychological mediation framework, the present study examined the underlying psychological processes through which HIV stigma from the public and within the gay community influences the mental and social health of MSM living with HIV. Findings from 206 Chinese MSM living with HIV in Hong Kong indicated that negative self-concept, maladaptive coping, and peer isolation mediated the effect of HIV stigma on mental and social health. The study revealed the cognitive, regulatory, and interpersonal processes underlying HIV stigma and health. Feeling intense HIV stigma from the public and within the gay community may render MSM living with HIV more vulnerable to negative self-concept, maladaptive coping, and peer isolation, which contribute to poor mental and social health. To combat prejudice and discrimination against people living HIV, stigma reduction initiatives should be implemented not only in the public, but also in the gay community. Cognitive-behavioral interventions can also be used to restructure negative self-beliefs and build adaptive emotion regulation skills, which can improve stigma-related health outcomes among MSM living with HIV.