The purpose of this current study was to investigate the reasoning stages of in-service middle and high school mathematics teachers in geometry. There was a total of 148 in-service middle and high school mathematics teachers involved in the study. Participants’ geometric reasoning stages were determined through a multiple-choice geometry test. The independent samples t-test with α = 0.05 was used in the analysis of the quantitative data. The study demonstrated that the in-service middle and high school mathematics teachers showed all the van Hiele levels, visualization, analysis, ordering, deduction, and rigor, and that there was no difference in terms of mean reasoning stage between in-service middle and high school mathematics teachers. Moreover, there was no gender difference found regarding the geometric thinking levels.