The purpose of the current study was to evaluate the impact of co-taught integrated STEM methods instruction on preservice elementary teachers' self-efficacy for teaching science and mathematics within an integrated STEM framework. Two instructional methods courses (Elementary Mathematics Methods and Elementary Science Methods) were redesigned to include STEM integration components, including STEM model lessons co-taught by a mathematics and science educator, as well as a special education colleague. Quantitative data were gathered at three time points in the semester (beginning, middle, and end) from 55 preservice teachers examining teacher self-efficacy for integrated STEM teaching. Qualitative data were gathered from a purposeful sample of seven preservice teachers to further understand preservice teachers' perceptions on delivering integrated STEM instruction in an elementary setting. Quantitative results showed a significant increase in teacher self-efficacy across all three time points. Item-level analysis revealed that self-efficacy for tasks involving engineering and assessment (both formative and summative) were low across time points, while self-efficacy for tasks involving technology and flexibility were consistently high. Qualitative results revealed that the preservice teachers did not feel adequately prepared by university-level science and mathematics courses, in terms of content knowledge and integration of science and mathematics for elementary students.