This study explored the research question: How can project-based inquiry (PBI) in STEM teaching be designed, planned, and implemented in two preschool settings? A case study was conducted, using a collaborative action research approach. This research was conducted with 11 teachers and 300 children from 11 classes at two preschools in Taiwan. The data sources included research logs, observations, interviews, and documents. The collected data were analyzed qualitatively (e.g., iterative coding). The results indicate that PBI in STEM teaching can be integrated successfully into preschool curricula. Children can engage enthusiastically in STEM activities in a well-planned learning environment. We provide some suggestions for readers who have an interest in STEM teaching or research at the preschool level.