Spatial thinking is fundamental to the practice and theory of geography, however there are few valid and reliable assessment methods in geography to measure student performance in spatial thinking. This article presents the development and evaluation of a geospatial thinking assessment instrument to measure participant understanding of spatial relations within a geographic context. The instrument is a test consisting of thirty question items, with the purpose to help identify the knowledge sets, thinking skills, and characteristics typical at different levels of competency. The performance score is used to classify participants along an expertise continuum, novice to expert. Generally, students performing at the expert level consist of graduate students while novices are at the grade nine level, although several anomalies are discussed. The broad instructional application of this assessment is to benchmark student performance and the level of understanding of geospatial concepts. (Contains 5 tables, 3 figures, and 4 notes.)