Including systems thinking in science education helps students understand the complex global problems of the present era. The study aimed to trace and evaluate the system thinking (ST) skills in K-8 science curricula, with a focus on sustainability-related subjects and units. Firstly, the authors reviewed the related literature on the systems thinking models and identified the components of systems thinking skills needed to evaluate the science curricula. Secondly, they developed a systems thinking rubric with two parts. The first part included eleven systems thinking components, definitions, key concepts, and sample learning objectives. The second part contained four assessment criteria to evaluate the ST levels (novice, beginning, intermediate and advanced) in the science curricula and science textbooks. Finally, the authors pilot-tested the rubrics to analyze the Turkish K-8 science curricula and textbooks in terms of ST skills. The results revealed that science curricula and textbooks included nine out of eleven systems thinking components; however, they were mostly at the beginning level. This study suggests that the integrated rubric is a valid and reliable tool to evaluate the systems thinking components, and science educators can use it to decide how to integrate ST skills into science curricula.