The aim of this study is to develop chemistry students' critical thinking skills using a STEM-Project-Based-Learning (STEM-PjBL) practice. Students developed soap and detergent waste treatment equipment to solve an environmental problem related to acid-base chemistry topics. Twenty-five grade 11 students at an Islamic boarding school in Banten Province, Indonesia, participated in this study. Qualitative methodology was used to gather data from a variety of sources such as classroom observation, interviews, reflective journals, and a critical thinking skills test. An analysis of students' critical thinking skills referred to the following five indicators: identify the question at issue, conceptual understanding, idea connections, assumptions, and inferences with five achievement levels. The study revealed that students reached a mastering and competent level, indicated by their ability to: ask questions and respond critically to a problem presented, use proper and adequate understanding of chemistry concepts, make connections between the issues encountered and the concepts studied; test assumptions accompanied by evidence; and deliver clear and logical conclusions. Therefore, the STEM-PjBL model can be considered an effective way for students to develop their critical thinking skills in chemistry and to support their active involvement in environmental education. [ABSTRACT FROM AUTHOR]