Background: Most bullying incidents occur in the presence of bystanders, with few choosing to intervene. Therefore, the development of a valid instrument to measure individuals' willingness to intervene in bullying is warranted. Aims: This study aimed to develop as well as validate a self-reported willingness to intervene in bullying scale (WIBS) for secondary school students. Samples: Two junior high school students' samples (N = 553; N = 950) in Taiwan were collected for both scale revision and scale validation. Methods: This study examined whether 'perceived severity of bullying' and 'self-efficacy of intervention' were important attributes of the willingness to intervene in bullying. The partial credit model (PCM) and the model with internal restriction of item difficulty (MIRID) were utilized to fit the data. Results: The WIBS had good model-data fit with both the PCM and the MIRID, and it suggested (via the MIRID) that 'perceived severity of bullying' and 'self-efficacy of intervention' are important components of the willingness to intervene in bullying scenarios, although the latter component had greater weight than did the former. Moreover, the willingness to intervene was related to the pro-victim attitude and self-reported defending behaviours. Conclusions: Students' willingness to intervene in school bullying situations could be explained by their self-efficacy in stopping bullying and their perceived severity of bullying incidents. Therefore, educators and researchers should attempt to raise students' self-efficacy regarding intervention and their perceived severity of all kinds of bullying to promote their willingness to intervene in school bullying situations.