The Teacher as Partner is an approach to inquiry-based science teaching, where the teacher works as the students' partner. Prior research suggests that it offers an opportunity to model inquiry actions and voice their often tacit rationale. It also enables learners to witness the difficulties and fallibility that even experts can experience, which in turn, can foster self-efficacy. We report on a quasi-experiment comparing 3 (experimental) Teacher as Partner classes, and 3 (control) classes working on the same inquiry unit as routinely taught. Experimental classes showed higher pre-to-post gains on both inquiry skills and scientific self-efficacy, with a medium-to-high effect size. This suggests that the Teacher as Partner is effective in supporting the development of both scientific practices and self-efficacy.