Background: Inclusive education can be of great benefit to the development of children and students with special education needs. Nonetheless, regular in-service teachers across several grade levels express their concerns over managing and educating students in need of special supports who are included in classrooms along with their peers. Inclusion-focused training has been suggested as a way to promote teachers’ positive attitudes towards the education of students with different abilities. Thus, this systematic literature review attempted to investigate the effect of in-service training on in-service teachers’ attitudes towards inclusion.

Method: The search was carried out through electronic databases including ERIC, PsycINFO, Web of Science, and Google Scholar limited to studies published between 1990 and 2020. The quality assessment process was conducted utilizing CASP checklist for randomized control trials.

Results: In total, 12 studies were found eligible to include in data synthesis. The quality assessment showed that most studies were of moderate quality concerning study design and generalizability of results. Nine studies reported that in-service training programs had an effect on teachers’ attitudes towards inclusion.

Conclusion: This study provides an overview of the effect of in-service training on regular classroom teachers’ attitudes across grade-levels. Although the teachers’ attitudes towards inclusion appear to be changed during the intervention process, long-term effects also need to be assessed. The study indicates that system-level supports and resources in the form of in-service training are beneficial for promoting inclusive attitudes among regular classroom teachers.