This article presents our work in translating the Mathematics Teaching Efficacy Beliefs Instrument (MTEBI) from English to Thai and our resulting investigation of validity with Thai preservice teachers. The translation process occurred over several meetings between two U.S. mathematics educators and one Thai mathematics educator. To check for reliability the instrument was translated into Thai, back-translated into English, and then cognitive interviews were conducted with native Thai speakers to check for accuracy, meaning, and readability. We used the newly translated Thai-Mathematics Teaching Efficacy Beliefs Instrument (T-MTEBI) to measure teacher efficacy beliefs as they related to Thai preservice mathematics teachers. Eight of the questions measure Mathematics Teaching Outcome Expectancy (MTOE). The mean of the scores on these questions was computed to form a MTOE score for each student. The remaining 13 questions measure Personal Mathematics Teaching Efficacy (PMTE). The mean of these scores was computed to obtain the PMTE score for each student. The mean of all 21 questions was computed to find an overall efficacy score for each student. The results of this study showed that the newly constructed T-MTEBI produced reliability and validity measures comparable to the original MTEBI (Enochs, Smith, & Huinker, 2000).