The UCLA Loneliness Scale, with different short versions, is widely used to assess levels of loneliness. However, whether the scale is valid in assessing loneliness among sexual-minority men is unknown. Additionally, it is unclear whether the 8-item and 3-item short versions are comparable to the full 20-item version. The present study compared the validity of the three versions of the UCLA Loneliness Scale (i.e., 20-item, 8-item, and 3-item versions) among gay and bisexual men in Taiwan. The participants comprised 400 gay and bisexual men in Taiwan who completed a cross-sectional online survey, which included the UCLA Loneliness Scale, Center for Epidemiological Studies Depression Scale (CES-D) and State–Trait Anxiety Inventory (STAI). Confirmatory factor analysis was used to evaluate factorial validity. Convergent validity was examined between the three versions of the UCLA Loneliness Scale and the CES-D and STAI. Known-group validity was investigated with participants’ sexual orientation and educational levels. The unidimensional construct was supported in all three versions of the UCLA Loneliness Scale tested in the present study. Convergent validity was supported as the level of loneliness was correlated with the level of depression and anxiety for all three versions. There were no significant differences between gay and bisexual men, although significant differences were found across different educational levels. The study confirmed that all three versions of the UCLA Loneliness Scale were comparable with satisfactory reliability and validity in Taiwanese sexual-minority men.