This paper reports our efforts to develop a measure of pre-service elementary teachers' geometry knowledge for teaching 2-dimensional (2D) shapes and to evaluate the psychometric properties of this measure. Specifically, the GKT-2D scale was designed to assess pre-service elementary teachers' geometry content knowledge, knowledge of geometry and students, knowledge of geometry and teaching in relation to 2D shapes, based on the van Hiele theory and mathematical knowledge for teaching framework. Using 307 pre-service elementary teachers' responses on the instrument, we examined item- and scale-level reliability and hypothesized factor structure of the instrument. The results suggested that the GKT-2D scale is a reliable and valid measure of the three facets of pre-service elementary teachers' geometry knowledge for teaching 2D shapes, with some limitations further addressed. This paper offers directions for future research in evaluating content-specific knowledge for teaching mathematics to unpack the complex relationship between teacher knowledge, teaching quality, and student learning.