Integrating science, technology, engineering, and mathematics (STEM) education in physical education (PE) has received increased attention. PE teachers' past behaviors and behavioral intentions of integrating STEM education in their teaching were investigated in this cross-sectional survey via the lens of the theory of planned behavior (TPB). Primary and secondary school PE teachers (n = 165) completed a survey that measured their behaviors and intentions surrounding STEM. The results indicated that only 19.4% of the PE teachers sometimes or often integrated STEM education in their teaching. Attitudes, subjective norms, and perceived behavior control positively predicted PE teachers' behavioral intentions, which was consistent with the TPB. These findings support the utility of TPB in understanding PE teachers' intentions to integrate STEM education in PE classrooms. The findings also provide the first evidence-based data for policymakers to promote STEM education in PE.