Much attention has been given to the development and validation of measures of growth mindset and its impact on learning, but the previous work has largely been focused on general measures of growth mindset. This research was focused on establishing the psychometric properties of a reading mindset (RM) measure among a sample of upper elementary school students and validating the measure through its relations with standardized measures of word reading and comprehension. The RM measure was developed to capture student's beliefs about their ability, learning goals, and effort during reading. Item response theory was used to select items that optimally measured the RM measure from a pool of existing items from previous research. The final five-item RM measure predicted reading comprehension outcomes above and beyond the effects of word reading, indicating that this measure may be an important tool for diagnosing noncognitive areas of improvement for developing readers. The implications, limitations, and future directions for expanding upon the measure were discussed.