Abstract: Undetectable = Untransmittable (U=U) is a key message emphasizing that antiretroviral therapy suppresses HIV and prevents its sexual transmission. However, dissemination of U=U varies among health care providers, potentially leading to knowledge gaps among patients. Little research exists on the understanding of U=U among active duty men who have sex with men (MSM) in the U.S. military. Our cross-sectional, online study examines 222 active duty MSM to determine prevalence of accurate knowledge of U=U and demographic predictors of misinformation. Participants received a pre-exposure prophylaxis (PrEP) overview and were asked to indicate if the statement "Undetectable equals Untransmittable" was true or false. Although the majority accurately understood U=U (70%; n = 156), approximately 30% did not ( n = 66); a binary logistic regression revealed lower U=U understanding among White, bisexual, unmarried, and Marines/Navy participants. Standardized education on U=U is crucial for resolving knowledge gaps and combating stigmas surrounding HIV treatment.