This research presents the outputs of problem-solving based STEM education activities applied in an undergraduate mathematics course. An experimental study using a randomized pretest-posttest control group design was carried out to examine the development of elementary pre-service teachers in the fields of mathematics achievement, belief about mathematical problem solving, STEM awareness, and problem-solving skills. Outcomes revealed that pre-service teachers in the experimental group (i.e. those taking the STEM course) showed a greater improvement in mathematics achievement in the areas of volume and surface area of solids, trigonometry, coordinate system, and STEM awareness than those in the control group after taking the course. Also, an increase in the problem-solving skills of pre-service teachers was found. The implications of the findings and the development of elementary pre-service teachers were discussed.