One of the institutionalized collaborative practices among teachers and other educators is the conduct of lesson study (LS). Considering the growing interest in the potential of LS to improve the quality of education through collaboration, it is worth exploring its potential contribution to enhancing the teaching competencies of teachers. This study determined the effects of LS on the teaching competencies as perceived by preservice and in-service teachers in mathematics and special education. A quasi-experimental study was undertaken in two selected middle schools (years 7–10). The participants’ perceived teaching competencies before and after the four LS cycles were determined using a survey questionnaire with indicators grouped into four dimensions: concept of diversity, teaching methodology, support services, and community participation. The Wilcoxon Signed-Rank test demonstrated a statistically significant large difference ( r  = .85, Z  = 3.29, p  < .001) between the pre-survey ( Mdn  = 4.08) and post-survey ( Mdn  = 4.34) ranks of the experimental groups. In contrast, the teaching competency gains of participants in the control group were statistically small and insignificant ( r  = .05, Z  = 0.14, p  = .889). Qualitative data obtained through observations, document analyses, and interviews further demonstrated that LS provided opportunities for enhancing the experimental groups’ understanding of classroom diversity, inclusive pedagogical content knowledge, and teaching math skills. Moreover, this study revealed that LS participation fostered increased self-esteem, confidence, and a sense of ownership and accountability among the LS participants.