Self-efficacy, or one's belief in their own ability to perform a task, is often investigated as a predictor of academic persistence and performance. The Math Your Earth Science Majors Need project developed the Geoscience Mathematics Self-Efficacy Scale (GeoMSES) that builds on prior work in measurement of self-efficacy for mathematics by focusing specifically on students' capacity to apply mathematical skills to typical problems encountered in majors-level undergraduate geoscience courses or in professional geoscience settings. The scale was developed as part of program evaluation research for a set of 14 co-curricular math and statistics modules designed to augment existing geoscience curricula. This 18-item tool was validated using data from students at 20 different institutions. The data had high internal reliability and stability and items were highly intercorrelated, yet not redundant. Our results show that the scale consistently measures student confidence across diverse groups, scores correlate with actual math performance, and items can be used individually to assess specific skills (e.g., vectors or linear regression) or as a full set for program evaluation. The GeoMSES can augment or serve as a less threatening alternative to traditional achievement measures and performance-based rubrics. Instructors and researchers can use this tool to identify "confidence gaps," tailor their teaching strategies, and evaluate the impact of math-integrated geoscience curricula.