Elementary teachers often feel underprepared to teach integrated STEM (iSTEM) and describe their preservice teacher preparation as ineffective. The purpose of this study is to investigate the extent to which semester-long STEM methods courses influenced elementary preservice teachers’ (PSTs) iSTEM teaching self-efficacy and identify how the sources of self-efficacy influenced their beliefs and confidence in teaching iSTEM. Participants included 131 PSTs at a large midwestern research-intensive public university in the United States. Quantitative data sources included the Self-Efficacy for Teaching Integrated STEM instrument administered as a pre-and post-test. The qualitative data collection included two semi-structured interviews with 10 selected participants. Findings suggest that PSTs experienced growth in iSTEM teaching self-efficacy through their STEM methods coursework. Sources that emerged as contributors toward enhanced confidence to teach iSTEM were enactive mastery, emotional arousal, vicarious experiences, and verbal persuasions. The implications for preservice teacher preparation programs and future research on iSTEM teaching self-efficacy are discussed.