The aim of the study was to examine the effectiveness of the Discovery Learning Module on Geometry (D-Geometry Module) in improving reasoning skills, communication, and self-confidence in mathematics among secondary school students. The research design was a quasi-experimental with a non-equivalent control group with pre-test and post-test design, where 128 students were grouped according to gender and the group of the intervention as well as a traditional group. Mathematical reasoning skills, communication, and self-confidence instruments were administered towards the students involved. Two-way MANOVA analysis showed that there was a main effect of the grouping. The finding showed that the integration of learning theories and discovery learning in Geometry could improve students' reasoning, mathematical communication, and self-confidence. Students' engagement is more dominant through the Discovery Learning Module rather than traditional learning. Therefore, the D-Geometry module should be used as a teaching resource for teachers, students, and curriculum-2013 (K-13) to facilitate classroom instruction.