Environmental Education (EE) is a learning process that provides necessary knowledge, attitudes, and skills toward the achievement of Sustainable Development Goals (SDGs). Critical Thinking Skills (CTS) is one of the vital Higher Order Thinking Skills (HOTs) necessary to acquire by the young generation to face the challenges of 21st-century education. Countries worldwide are focusing on the inclusion of EE and revising their curriculum and textbooks to focus on students' attainment of CTS achievement. As the focus of developing the CTS increases, the CTS assessment has become more vital nowadays. The primary purpose of the current study was to develop and validate a test to measure students' critical thinking skills in a specific subject of environmental education. A Critical Thinking Skills Test of Environmental Education (CTSEE) was developed and validated with 444 secondary (grade 8) level students of Bangladesh. The developed test comprises 27 multiple-choice type items under three skills-conclusions, inferences, and identifying bias. Results were analyzed to measure the quality of individual test items, establish reliability estimates, and ensure the developed instrument's validity. Results showed substantial validity and reliability of the test items, along with a moderate level difficulty. It is hoped that the developed test contributes to the future educators and researchers by filling up the gap of a systematically designed and valid instrument for measuring the CTS among secondary level students in different environmental issues.