Project-based learning (PjBL) is an approach often favored in STEM classrooms, yet some studies have shown that teachers struggle to implement it with academic rigor. This paper explores the relationship between PjBL and rigor in the classrooms of ten STEM-oriented high schools. Utilizing three different data sources reflecting three different perceptions--student surveys, teacher logs, and classroom observations--the study examines the extent to which PjBL and rigor co-occur. Across all three measures, the results show that use of PjBL is associated with higher levels of rigor. However, the study also shows that academic rigor can be present in the absence of PjBL, and that PjBL can be implemented with low levels of rigor. The paper concludes with implications for practice.