In the present study we aimed to develop a multidimensional test assessing high school students' knowledge about triangles, and then to determine the validity evidence for it based on the internal structure and relations to another variable and its reliability. The test developed was administered to 557 tenth grade students. To assess the validity evidence based on the internal structure, the data were analyzed using confirmatory factor analysis, inter-dimension correlations and two-way MANOVA across gender and school type. For validity evidence based on relations to another variable, the test scores were associated with previous semester geometry grades at the tenth grade level. Reliability was assessed using Cronbach alpha and item-total correlations to report internal consistency. The resulting instrument including 24 questions showed adequate validity evidence based on the internal structure and relations to another variable, as well as good reliability. This indicates that the results produced by the instrument are valid and reliable. So, the geometry knowledge test about triangles is well-suited for use in research and classrooms.