STEM education’s focus on interdisciplinary learning presents an ideal platform for integrating education for sustainability (EfS) in K–12 curricula. Understanding STEM educators’ sustainability knowledge and their current incorporation of sustainability topics into STEM lessons is essential for further advancing EfS. To investigate these aspects, a mixed-methods study, involving both surveys and interviews, was conducted on STEM educators in Pennsylvania, United States. The results showed that most STEM educators surveyed had a simplified and incomplete understanding of sustainability, which aligns with the findings of international research. Despite the assumption that greater sustainability knowledge and perceived importance of EfS lead to teaching more about sustainability, no statistically meaningful correlation was found among these variables, indicating additional obstacles to integration. Watersheds, wetlands, and environmental experiences were identified as the least integrated topics in STEM curricula, suggesting a need for targeted efforts to enhance these areas.