Self-efficacy plays a key role in determining teaching practices. Still, concerns regarding the low science self-efficacy beliefs held by elementary teachers have raised questions about the effectiveness of teacher preparation programs. Previous research has been restricted to labeling preservice elementary teachers as having high, medium, or low science teaching self-efficacy. Few extended efforts have been made to clearly distinguish preservice elementary teachers’ subject-specific lack of confidence in teaching science. The present study aimed to investigate preservice elementary teachers’ subject-specific self-efficacy in teaching science using a mixed-method research design. The participants included 55 preservice elementary teachers at a university in the United States. Quantitative data were collected using the Beliefs About Teaching instrument, and interviews were used to collect qualitative data. The results showed that the participants had the highest self-efficacy level in biology, followed by earth science, chemistry, and physics. Four themes emerged from the participants’ reasons for feeling more confident in teaching biology concepts than physics: education experience, teacher experience, subject comprehension, and subject relevance. This study highlights the essential role played by teacher preparation programs in providing ample opportunities for preservice elementary teachers to develop a strong understanding of the content and teaching methods of all science subjects.