Background Most Thai transgender people (TG) do not use gender-affirming hormone therapy (GAHT) under medical supervision. Aim To understand the current real-world characteristics of GAHT among TG. Methods A cross-sectional survey was conducted using an online questionnaire between September and December 2020. TG, who resided in Thailand for more than 1 year, were included. Self-reported demographic data and characteristics of GAHT were obtained. The reported GAHT were compared to the reference regimen recommended by the 2017 Endocrine Society Clinical Practice Guideline. Outcomes The characteristics of GAHT used among TG and factors associated with hormonal use outside the reference regimen were reported. Results A total of 401 TG were included in the analysis. Of these, 249 (62%) were transgender men (TM). Most TM (81%) and transgender women (88%) were using GAHT. Only 297 TG provided a complete hormone regimen. A total of 224 TG (75%) used GAHT outside the reference regimen. The main reasons in TM were using intramuscular testosterone with a higher dose per injection and less frequent intervals. In transgender women, using oral contraceptive pills and cyproterone acetate 12.5 mg/d were the two most common reasons. A univariate analysis revealed factors associated with hormonal use outside the reference regimen, including age at a survey participation (OR 1.04, 95%CI 1.00–1.08, P = .047), age at hormone initiation (OR 1.04, 95%CI 1.01–1.08, P = .015), TM (OR 2.08, 95%CI 1.22–3.56, P = .007) and using GAHT, not under medical supervision (OR 1.78, 95%CI 1.04–3.05, P = .037). The multivariate analysis showed that only living outside the capital city was solely statistically significant (AOR 1.77, 95%CI 1.02–3.05, P = .041). Clinical Implications Enhancing health literacy in GAHT among TG is crucial, especially TG not living in the capital city. Strengths and Limitations This study demonstrates a current real-world practice of GAHT use among TG, both under and not under medical care. However, the causality could not be concluded due to the nature of the cross-sectional observation study, and results come with a recall bias. Conclusion There is a high prevalence of GAHT use outside the reference regimen. The only factor associated with hormonal use outside the reference regimen is living outside the capital city.