Background/purpose: The purposes of this research are to develop a reliable and valid assessment tool for measuring systems thinking skills in upper primary students in Thailand and to establish a normative criterion for evaluating their systems thinking abilities based on educational standards. Materials/methods: The study followed a three-phase data collection process, involving a total of 1,000 students from 17 schools in the Thai educational context. The sample was divided into three groups: 70 students for item analysis difficulty (p) and discrimination (r), 430 students for construct validation using Confirmatory Factor Analysis (CFA), and 500 students for T-score ranking and normative criteria development. The Systems Thinking Skills Assessment for Upper Primary Students was developed as a multiple-choice test. Results. The results showed that the assessment had strong content validity (IOC = 0.60-1.00), appropriate item difficulty (p = 0.30-0.66), acceptable discrimination indices (r = 0.20-0.45), and high internal consistency (KR-20 = 0.82). The CFA confirmed that the assessment model was well-aligned with empirical data, with X[superscript 2] = 0.24, p = 0.62, RMSEA = 0.00, CFI = 1.00, GFI = 1.00, and AGFI = 1.00. Additionally, T-score ranking was established, providing normative criteria for interpreting students' systems thinking performance. Conclusion: This study was conducted to develop and validate a reliable assessment tool for measuring systems thinking skills in upper primary students. The research followed a three-phase data collection process, involving content validation, construct validation through Confirmatory Factor Analysis (CFA), and the establishment of normative criteria using T-score ranking.