This study aims to develop and validate the Scale of Self-Regulated Language Learning (S2RLL), intended to measure high school students' self-regulated language learning skills such as grit, self-efficacy, goal orientations, and language learning strategies. A total of 1281 Korean high school students participated in this study. Various evidence for S2RLL validity was collected from these participants. Results from a field expert panel rating agreement and the Rasch analysis offered evidence for content validity. The Rasch analysis also provided evidence for response validity. Internal structure-related evidence for validity was obtained by applying both exploratory and confirmatory factor analysis and calculating various reliability indices. Lastly, evidence for criterion validity was gained by comparing results from S2RLL with those of other instruments that measure constructs similar to S2RLL. We found that S2RLL items appropriately reflected self-regulated language learning skills. From a psychometric perspective, exploratory and confirmatory factor analyses of shared item variance supported the valid internal structure of S2RLL. Moreover, the Rasch analysis offered evidence for the appropriateness of the 5-point Likert scale used by respondents to respond to S2RLL items. Finally, the sub-scales of S2RLL showed moderate to high correlations with those of other scales related to self-regulation. Thus, S2RLL has the potential to help evaluate applicants' SRL for second language learning. In addition, we make several suggestions for future research in this area.