In the context of Nepal, most mathematics teachers at the secondary level are still using conventional teacher-centred pedagogy. It is assumed that teachers' pedagogy might enhance learners' engagement in learning mathematics and build conceptual understanding. Project-based learning (PBL) is a student-centred pedagogy that can remove misconceptions about content knowledge, enhance engaged learning, and support teachers in developing pedagogical skills. The study tried to reform the pedagogical practice of teachers using conventional teacher-centred pedagogy in teaching mathematics. The action research was conducted in four secondary schools in Nepal by five secondary mathematics teachers in two cycles, each seven days. The study focused on the implementation of PBL in secondary-level schools, the impact of PBL on students' activities, and professional development. The seven-day project was prepared based on a 3-day workshop for teachers on the surface area of prism and pyramid of grade ten mathematics. The action research was conducted in four different school settings, and students' activities were observed. The study collected qualitative data through interviews (baseline and post), observation, and teacher reflection notes. Text data were analyzed thematically for meaning-making and finding. The study's significant findings were engaged learning, inclusive education, and contextualization of mathematics. Likewise, students' creativity was observed during the action research, and teachers were supported in reforming their pedagogy with new experiences through PBL. The study might support other teachers in implementing PBL for student-centred pedagogy and teachers' professional development.