The aim of this study was to determine the effect of Lesson Study (LS) on pre-service secondary teachers’ technological pedagogical content knowledge (TPACK) for teaching mathematics with The Geometer’s Sketchpad (GSP). Lesson Study is a Japanese model of teacher professional development in which small groups of teachers collaboratively plan, teach and revise a lesson to improve the quality of their teaching as well as to enrich students’ learning experiences. GSP is a dynamic geometry software program for constructing and investigating mathematical objects that enhances the teaching and learning of geometry and many other areas of mathematics. A single-group pretest-posttest design was employed to examine whether there was a significant difference in the pre-service secondary teachers' TPACK for teaching mathematics with GSP after engaging in LS which was incorporated into the mathematics teaching methods course during the first semester of the 2011/2012 academic session in a public university. Forty-six pre-service secondary teachers who enrolled in the course completed the Survey of Pre-service Secondary Teachers' TPACK for Teaching Mathematics with GSP before and after engaging in LS. The results of the paired-samples t-test showed that there was a significant difference in the pre-service secondary teachers' TPACK for teaching mathematics with GSP after LS.