Young children, ages 5-6 years, develop first beliefs about science and themselves as science learners, and these beliefs are considered important precursors of children's future motivation to pursue science. Yet, due to a lack of adequate measures, little is known about young children's motivational beliefs about learning science. The present two-part study explores the motivational beliefs of young children using a new measure--the Young Children's Science Motivation (Y-CSM) scale. Initial measurement development involved a thorough literature review of existing measures, and an extensive piloting phase until a final instrument was reached. To establish scale reliability, measurement invariance as well construct and criterion validity, the final instrument was administered to a new sample of 277 young children, age 5-6 years, in northern Germany. Results reveal that children's motivational beliefs can be empirically differentiated into their self-confidence and enjoyment in science at this young age. Older children were more motivated in science, but no significant gender differences were found. Importantly, children in preschools with a science focus reported significantly higher science motivation. This finding stresses the importance of early science education for the development of children's motivational beliefs science.