Teaching assistants (TAs) play an influential role in primary and secondary schools. But there is an absence in literature about the TA’s efficacy, and to date no instrument is available for measuring TA’s efficacy. The present study aims to develop and validate a scale (Teaching Assistant Efficacy Scale, TAES) for measuring TA’s efficacy on identified capabilities. A total of 531 teaching assistants from Hong Kong schools participated in the survey. The multidimensional Rasch model was used to analyse the data. The results revealed that a 5-dimension structure of TA’s efficacy was supported. The final 30-item version of TAES assesses TA’s efficacy on learning support, teaching support, behaviour management, cooperation, and administrative support. The Rasch reliabilities for all five dimensions were around 0.90. The 6-category response structure worked well for the scale. Further research was recommended to validate and test the robustness of the TAES both in Hong Kong and elsewhere.