Purpose: The purpose of this paper is to explore the coaching and mentoring supervision of the Lesson Study to enhance preservice mathematics teachers' research competencies. Design/methodology/approach: The researcher employed a multi-case study design to describe the internship experiences of practicing coaching and mentoring supervision while preservice teachers engaged in Lesson Study. A total of six preservice teachers were the samples, and they were teaching six lessons. A classroom observation evaluation form was used to collect data from 60 observers, with a proportion of 10 observers to each sample to assess and provide feedback on the samples' research competencies. This was followed by 12 individual, semi-structured interviews using interview protocol. The participants included 60 Lesson Study group members and six mentors from the three research schools, together with the samples' internship advisors from Suratthani Rajabhat University, totaling to 72 assessors. Data were examined using the thematic analysis method. Findings: The results of 60 observation evaluations revealed that coaching and mentoring supervision of Lesson Study can develop samples' research competencies in four capabilities, namely, instructional planning, classroom management, instructional delivery and professional development. The 12 interview results indicated that the six samples' research competencies were improved in these four aspects, namely, data collection and analysis, collaboration, reflection and pedagogical content knowledge. Practical implications: Mentoring and coaching supervision of Lesson Study holds great potential for collaborative professional development and continuous improvement of mathematics teachers' research competencies. Originality/value: This article suggests meaningful professional benefits of participation in mentoring and coaching supervision.