<p>The Undetectable equals Untransmittable (U=U) message improves health outcomes among people living with HIV (PLHIV) and reduces HIV-related stigma. Some PLHIV are unaware of U=U, and those who know about U=U sometimes perceive it as inaccurate. Healthcare providers (HCP) are an important source of information for PLHIV, but they don’t consistently or accurately communicate U=U with PLHIV. We conducted web-based surveys among PLHIV and HCP in Thailand from December 2021–August 2022 to assess awareness, belief and understanding of U=U. Binary logistic regression analysis assessed associations between U=U awareness, belief and understanding, and wellbeing (i.e. optimal physical, mental, sexual and overall health, and HIV-related worries) among PLHIV. HCP were asked how and how often they discussed U=U in their clinics. Logistic regression assessed associations between U=U awareness, belief, and correct communication among HCP. Among 172 PLHIV 86.0% were aware of U=U, of whom 88.3% understood it and 90.3% believed it. Awareness, belief and understanding of U=U were associated with improved wellbeing, e.g. undetectable viral load and not being worried about HIV transmission. Among 249 HCP, 91.6% had heard about U=U, of whom 84.4% understood it, 87.1% believed it, and 23.5% correctly communicated it (using ‘zero’ or ‘no’ risk). HCP who understood U=U or were confident in U=U communication were more likely to correctly communicate U=U. Gaps in U=U knowledge, belief and understanding still exist among PLHIV and HCP. More efforts to create awareness, belief and understanding are needed, along with interventions to increase confidence and correct U=U communication among HCP.</p>