© 2018, The Author(s).This study reports on the development and validation of Language Assessment Knowledge Scale, LAKS, which aims to measure teachers’ language assessment knowledge. The initial development of the scale included a thorough literature review, creating an item pool, and an initial expert opinion stage. The later process, which led to the development of the final form, consisted of several detailed stages including organizing meetings with practitioners in the field, gathering opinions of experts in language testing and assessment, and piloting the scale. At the end of this long-lasting validation process, LAKS with 60 items and 4 constructs (assessing reading, assessing listening, assessing writing, and assessing speaking) was completed by 542 EFL teachers working in higher education context. The statistical procedures during the analysis process included second-order confirmatory factor analysis, and Cronbach alpha for the reliability. The findings revealed that LAKS had a good model-data fit with the obtained factor loads, and Cronbach alpha coefficients were satisfactory. Concluding that LAKS can serve as a valid and reliable instrument to measure language teachers’ assessment knowledge, the study offers several suggestions for further research.