Feedback agency is a key concept in enhancing student writing performance. While growing attention has been paid to student feedback agency, existing research remains largely theoretical and qualitative. As a result, there is a lack of psychometrically supported instruments to measure this construct. To address this gap, the present two-phase study aimed to develop and evaluate a Student Feedback Agency Scale (SFAS), drawing on social cognitive theory. In the development stage, Principal Component Analysis (PCA) was conducted on a sample of 235 Chinese international postgraduate students. The results yielded a 23-item SFAS comprising six components: Action Taking, Goal Setting, Processing, Generating, Self-efficacy, and Seeking. Using an independent sample of 349 participants from the same population, Confirmatory Factor Analysis (CFA) was conducted in the evaluation stage. The results supported a good model fit (RMSEA = .055, IFI = .923, TLI = .908, and CFI = .922). Multi-group CFAs further confirmed the structural invariance across gender, academic level, and discipline. Overall, the findings provide psychometric evidence to support the interpretation and use of the SFAS scores to measure student agency in writing feedback processes. Based on these results, the factor structure and subscales of the SFAS are discussed, and implications are outlined.