Research suggests that transgender (trans) populations, a diverse group of persons whose gender identity and/or expression differ from societal expectations associated with their sex labelled at-birth, have high rates of non-HIV sexually transmitted infections (STIs). Untreated non-HIV STIs can contribute to negative health outcomes and community transmission. Therefore, access to non-HIV STI testing is of critical importance to mitigate morbidity and promote public health. Of the published research focused on non-HIV STI testing among trans people, studies are limited in population focus (almost exclusively trans women), study design (predominantly qualitative), and geographic context (large urban centers/known trans-friendly cities). Moreover, studies to-date have excluded nonbinary persons (those who do not identify exclusively as men or women), though this group comprises 1/3 of all trans persons in the U.S. To address these gaps, our team will examine whether sexually active trans and nonbinary (TNB) populations are characterized by gender-based differences in their rates of non-HIV STI testing and barriers and facilitators to non-HIV STI testing. We will conduct an explanatory sequential mixed-methods study using secondary quantitative data from the 2018 Michigan Trans Health Survey (MTHS) and a primary qualitative study of TNB persons using online focus groups. First, we will determine gender-based differences in a) rates of lifetime non-HIV STI testing history (never, ever) and b) time since last non-HIV STI test (never, past year, 1-5 years, >5 years) among sexually active TNB persons. Then, informed by an integrated conceptual framework drawing on social ecology, minority stress, and gender affirmation, we will examine whether individual (e.g., race/ethnicity), medical (e.g., hormone therapy); social (e.g., provider discrimination based on gender identity); and structural (e.g., clinic gender affirmation) factors are uniquely associated with lifetime non-HIV STI testing history, and whether gender identity moderates the association between lifetime non-HIV STI testing history and statistically significantly associated independent variables. Finally, we will collect and analyze primary qualitative data from TNB persons to further understand their barriers and facilitators to non-HIV STI testing. This study will inform future research focused on developing, evaluating, and implementing interventions which increase non-HIV STI testing among TNB populations, filling critical gaps in research specifically to the needs of overlooked groups of trans men and nonbinary people.