Background Over 50% of HIV infections in Thailand annually occur among men who have sex with men (MSM) and transgender women who use online applications to meet their sexual partners. We conducted a cross-sectional study assessing undetectable=untransmittable (U=U) understanding, pre-exposure prophylaxis (PrEP) awareness, sexual behaviours and factors associated with HIV knowledge among users of Hornet in Thailand.

Methods From November 2019 to January 2020, a survey was conducted using convenience sampling on Hornet in Thailand. HIV literacy was assessed via 22 questions, and multivariable linear regression was performed.

Results 960 responses were assessed; median age was 34 years, the majority were MSM (80.4%), Thai (83.8%), had at least bachelor’s degree (74.9%). Regarding the risk profiles, 39.1% reported inconsistent condom use, 15.0% used amphetamine-type stimulants, 56.9% had not taken PrEP in the last six months and 20.5% never had an HIV test. Only 22.8% thought that U=U was completely accurate. Lower HIV knowledge was associated with being from Africa (β −8.13, 95% CI −14.39 to –1.87), age of 25 years or younger (β −2.6, 95% CI −4.37 to –0.82), education less than bachelor’s degree (β −2.58, 95% CI −3.98 to –1.19), having more than one sexual partners (β −2.41, 95% CI −4.13 to –0.69), paying three or more people to have intercourse (β −2.5, 95% CI −4.26 to –0.74), not knowing one’s HIV status (β −3.56, 95% CI −5.45 to –1.68) and not answering about previous PrEP use (β −4.11, 95% CI −7.86 to –0.36). Higher HIV knowledge was associated with being from Europe (β 2.54, 95% CI 0.46 to 4.61), the Americas (β 3.37, 95% CI 0.44 to 6.30) and previous PrEP use (β 2.37, 95% CI 0.94 to 3.81).

Conclusion Among Hornet users in Thailand, the U=U message was unclear, and PrEP use was suboptimal. Large HIV knowledge gaps and high-risk behaviours were concerning. Educational campaigns in online spaces are needed to promote awareness and HIV prevention strategies.