The purpose of the current study is to develop a mathematics self-report inventory (MSRI) to measure Turkish elementary students' mathematics expectancy beliefs and task values based on the expectancy-value theory of achievement motivation. In Study-1 (n = 1,315), exploratory factor analysis (EFA) and reliability analysis are used to evaluate the MSRI. EFA results suggest a five-factor model that consists of interest value, extrinsic-utility value, importance value, personal cost, and expectancy beliefs. In Study-2 (n = 1,343), confirmatory factor analysis (CFA) and convergent, discriminant, and subgroup validity results are also used to examine construct validity for the MSRI. The results indicate that (a) elementary students' scores on the MSRI did not significantly differ by gender; (b) elementary students' scores on the MSRI were significantly influenced by grade level; and (c) a significant, positive, and strong relationship existed between students' scores on the MSRI and students' achievement scores from the Level Determination Exam. The findings of this study showed that, with a few exceptions, the MSRI is a reliable and valid psychological tool for use with the elementary students in Turkey and may be regarded as beneficial in guiding subsequent study goals by explaining students' achievement-related beliefs grounded in the expectancy value theory.