Growth mindset in mathematics is an approach to teaching mathematics, believing that students' abilities can be developed with efforts, strategies, and support. With a sample of 178 middle school students, this study proposed a scale for mathematical mindset utilizing Boaler's positive norms in teaching mathematics and collected evidence for initial validation. The scale was unidimensional and reliable. Specifically, it was highly reliable for middle to low mathematical mindset levels. However, measurement precision was not as desirable in assessing high-level mathematical mindset. The convergent and discriminant evidence was established in relation to growth mindset, math anxiety, and math engagement. Item response theory analysis showed that the response categories properly gauged different levels of mathematical mindset. In sum, the results supported the scale to be a psychometrically sound measure of mathematical mindset. Implications for future studies and proper use of the scale were further discussed.