Student diversity not only brings about opportunities for creative teaching but also challenges for ensuring educational equity and the provision of high-quality teaching for all students from diverse backgrounds, especially those presently underserved by the educational system. The main goal of the study is to explore how a group of teachers enacted their beliefs of diversity in teaching and how their beliefs were shaped by a particular professional learning community—lesson study. This qualitative study is situated in two elementary schools (School A and School B) serving low-income neighborhoods in Singapore. We used multiple methods, including in-depth individual interviews, observations, and analysis of written reflections, to probe what the participant teachers thought about social diversity and its implications for teaching. It was found that: (1) teachers’ beliefs of diversity influenced their teaching strategies for working with students from diverse backgrounds; (2) Lesson study (as among other forms of professional learning communities) could shape teachers’ beliefs of diversity and eventually change their teaching practices. Certain mechanisms embedded in lesson study, such as examining student work with inputs from knowledgeable others can be further unpacked to sensitise teachers’ recognition of student differences in teaching. This study adds in-depth understanding on how lesson study could be employed for teacher professional learning and equity in education.