Computational thinking (CT) is a set of cognitive skills that every child should acquire. K-12 classrooms are expected to provide students opportunities (tasks) to think computationally. We introduce a CT competency assessment for middle school students. The assessment design process started by establishing a cognitive model of CT domain mastery, in which three broad skill types were identified to represent CT competency. After multiple-choice item prototypes were written, pilot tested, and revised, 15 of them were finally selected to be administered to 564 students in two middle schools in the Midwestern United States. Using a cognitive diagnostic scoring model, mastery classifications for each student were determined that can be used diagnostically by teachers as a pretest and, perhaps in the future, to compare the outcomes of CT instructional programs. The results inform an initial understanding of typical learning progressions in CT at the middle school level.