In the field of HIV prevention and care, most studies of HIV syndemic problems are cross-sectional, few differentiate by HIV transmission groups, and few focus on people living with HIV (PWH). We analyzed one-year longitudinal data of 692 sexually active PWH (heterosexual men [HM], heterosexual women [HW], and men who have sex with men [MSM]) in care from Brazil, Thailand, and Zambia. Syndemic scores (0-3+) included stimulant use, polydrug use, depression, alcohol use, and fear of discrimination. Overall, syndemic scores were associated with lower ART adherence over time, but this differed across sexual transmission categories. For HM and HW, those with 2 or 3+ syndemic problems had lower odds of ART adherence than those with none. However, for MSM, the association between syndemic scores and ART adherence was not significant. While syndemic problems generally predicted suboptimal ART adherence among PWH, the association appears nuanced across subgroups.