The purpose of this study is developing a valid and reliable scale intended to determine the irrational beliefs of students in mathematics. The study was conducted with a study group consisting of 700 students in 2015-2016 academic year. Expert opinions were received for the content and face validity of the scale, and the Exploratory Factor Analysis (EFA) and Confirmatory Factor Analysis (CFA) were applied. After the EFA was applied a structure was obtained consisting of 20 items, which explained 53.86% of the total variance and the four factors. The findings obtained from the CFA showed that the structure consisting of 20 items and the four factors related to the Irrational Beliefs in Mathematics Scale (IBIMS) had adequate consistency indices (x[superscript 2]/df = 2.50, RMSEA = 0.056, SRMR = 0.056, GFI = 0.92, AGFI = 0.90, CFI = 0.92, IFI = 0.90, PNFI = 0.76). The total internal consistency coefficient of the scale was calculated as 0.81, and the internal consistency coefficients of the items of "Finding Reasons," "Perfection," "Being Conditioned" and "Inclinations for Being Accepted" were calculated as 0.85, 0.78, 0.71 and 0.66 respectively. The test-retest measurement reliability was found to be 0.75. The discrimination of the items in the scale was made with the total corrected item correlation by comparing the 27% lower-upper group comparisons.