This research investigates how a lesson study (LS) on designing and implementing challenging tasks impacts Vietnamese high school mathematics teacher knowledge and beliefs. Its contribution highlights cultural considerations when adopting LS to the forefront to contextualize the impacts. The results show that the teachers developed their "specialized content knowledge" by attending to students' mathematics and creating cognitive conflicts building on student responses. The teachers changed their "curriculum knowledge" from "implementer" to "transformer," improved "knowledge about content and students" attending to difficulties and misconceptions, and enhanced their "knowledge about content and teaching" in ways they designed, sequenced, and evaluated approaches that fit student learning. Finally, they changed their beliefs about mathematics to a comprehensive view of knowledge, "mathematical proficiency," and "sophisticated beliefs" of teaching and learning. Discussion about the essence of LS when adopting it to different cultures is included.