Social media platforms offer the opportunity to develop online social networks. Use of these platforms has been particularly attractive to younger sexual and gender minority individuals as well as those living with HIV. This cross-sectional study examined the perceived level of social support and associations with social media use among youth and young adult cisgender men who have sex with men (MSM) and transgender (trans) women living with HIV and examined these associations by gender identity. The study drew from baseline data collected from 612 cisgender MSM and 162 trans women enrolling in one of 10 demonstration sites that were part of a Health Resources and Services Administration Special Projects of National Significance initiative. The individual projects were designed to evaluate the potential for social media/mobile technology–based interventions to improve retention in care and HIV health outcomes. The data used in this study came from baseline surveys completed when participants enrolled in a site between October 2016 and May 2018. Results demonstrated that a significantly greater proportion of MSM than trans women participants reported the use of social media platforms (e.g., Facebook: MSM = 86%, trans women = 62%; Instagram: MSM = 65%, trans women = 35%). Furthermore, increased social media use improved perceptions of social support only among MSM participants (direct adjusted OR = 1.49) and not trans women participants (gender identity interaction term adjusted OR = 0.64). These results revealed that MSM participants perceived greater social benefit from the use of social media platforms than trans women, which could be a result of generalized online transphobia experienced by trans women. More nuanced data on various social media platforms, that is, anonymous versus profile-based, and group differences, are needed to better understand how social media platforms can be best utilized to optimize health care outcomes among sexual and gender minority youth and young adults living with HIV.