Teachers’ intention to adopt and implement curriculum reform is an important matter for success in achieving standards for education. Science-Technology-Engineering-Mathematics practices advocated in the current curriculum. In this research pre-service math and science teachers’ intentions to implement STEM activities are investigated through the general framework of Theory of Reasoned Action. The questionnaires are formed by the researchers with the interviews and literature related to the theory. 260 pre-service math and science teachers from a state university in Turkey engaged in the research. The reliability and validity indexes of the questionnaires had evidence for the usability of the instruments. According to the results of the study, pre-service math and science teachers’ attitudes towards STEM activities, perceived behavioral control and subjective norms had significant contribution to their intentions to practice STEM in their classrooms. Behavioral beliefs including perceived behavioral expectancy and perceived behavioral values were effective for explaining pre-service teachers’ attitudes toward STEM. Subjective norms were significantly predicted by descriptive norms while injunctive norms did not significantly contribute to their formation. Perceived behavioral control is shaped by perceived difficulties rather than perceived behavioral convenience for STEM. Overall, the theory is effective and suitable in explaining pre-service teachers’ STEM intentions and pre-service teachers had high intention scores for implementing STEM in their classrooms. Further research may concentrate on improving in-service and pre-service teachers’ perceptions via effective implementations related to STEM both for their professional development and their students’ development.