Though men who have sex with men (MSM) are disproportionately affected by sexually transmitted infections (STIs), factors that impact STI testing adherence among non-single MSM remain under-explored. While being in a relationship per se does not necessarily increase one’s risk for STIs, certain behavioral risks and demographic factors may impact STI testing adherence. Through a sample of 296 non-single MSM located in the United States, we examined key behavioral and demographic factors and their associations with adherence to CDC’s STI testing guidelines. Overall, the results showed inconsistent STI testing adherence rates among divergent subgroups of higher-risk non-single MSM. First, non-single MSM who take PrEP were more likely to adhere to STI testing and showed significantly higher adherence rates than those who do not take PrEP, but adherence rates were not related to nor significantly different than those who reported extra-relational sex or condomless anal sex. Further, STI testing adherence was positively associated with having a shorter relationship length, identifying as non-White, and living in an LGBTQ+-friendly neighborhood. Practical implications and recommendations for clinical practices, persuasive messages, and promotion strategies are discussed.