Observation of different levels and types of organizational problems, such as school principals and teachers suing each other and conflicts between teachers, caused by union bias in schools in Turkey today, is the starting point of this study. This study, therefore, aimed to develop and validate a scale that helps determine union bias in schools. Participants of the study included teachers being a union member and working at a primary, middle or high school in the 2017-2018 and 2018-2019 academic years in Düzce, Turkey. During the data analysis, firstly, the first data set was examined in terms of the assumptions of the factor analysis and then Exploratory Factor Analysis (EFA) was applied. Confirmatory Factor Analysis (CFA) was performed on the second data set. Convergent validity of the scale was examined with item load values, mean-variance, and composite reliability coefficients. Discriminant validity was examined by the Fornell-Lacker criteria. Also, measurement invariance in gender groups was examined. The Cronbach Alpha and combined reliability coefficients were calculated to determine the scale's reliability. The Union Bias scale consists of 27 items and six dimensions. The explained total variance was 64%. As a result of the first and second order confirmatory factor analyses, it was revealed that the six-dimensional structure predicted Union Bias and the scale's structure did not differ in gender groups. The Cronbach's alpha value was 0.90 and the composite reliability was 0.96. As a result, the structure of the Union Bias Scale was concluded to be valid and reliable.