This study provides evidence of the validity and reliability of a self-efficacy questionnaire in English as a Foreign Language (EFL) in a Vietnamese sample of university students. A total of 656 non-English-major students completed the questionnaire in an online format. SPSS version 24, SPSS AMOS, and SmartPLS 3 were employed to analyze the data. A confirmatory factor analysis was carried out to assess the hypothesized structure model and several analyses such as composite reliability, Cronbach's alpha [alpha], and rho_A were conducted to evaluate reliability. The results showed that the questionnaire had a high reliability and adequate validity. All the values were satisfactory, and the model was confirmed. Therefore, the adapted questionnaire in this study can be applied to measure self-efficacy in EFL contexts as it gives valuable feedback to teachers and students to improve the quality of language teaching and learning.