Little is known about mobile application (app)-based behavior of men who have sex with men (MSM) in Thailand. A cross-sectional online assessment of app users in Bangkok found that more than a quarter have never tested for human immunodeficiency virus (HIV) and 1 in 3 never tested for sexually transmitted infections (STI). STI testing patterns and HIV testing frequency were highly associated with each other in multinomial logistic regression. In the midst of an escalating epidemic where HIV incidence among MSM is highest in Asia, apps can serve to engage those least likely to be reached by traditional methods of recruitment and outreach in Thailand.