The aim of this research was to examine the effect of Augmented Reality (AR) supported geometry teaching on students' 3D thinking skills. This research consisted of 3 steps: (1) developing a 3D thinking ability scale, (ii) design and development of an AR Geometry Tutorial System (ARGTS) and (iii) implementation and assessment of geometry teaching supported with ARGTS. A 3D thinking ability scale was developed and tested with experimental and control groups as a pre- and post-test evaluation. An AR Geometry Tutorial System (ARGTS) and AR teaching materials and environments were developed to enhance 3D thinking skills. A user study with these materials found that geometry teaching supported by ARGTS significantly increased the students' 3D thinking skills. The increase in average scores of Structuring 3D arrays of cubes and Calculation of the volume and the area of solids thinking skills was not statistically significant (p > 0.05). In terms of other 3D geometric thinking skills' subfactors of the scale a statistically significant difference was found in favour of the experimental group in pre-test and post-test scores (p < 0.05). The biggest difference was found on ability to recognize and create 3D shapes (p < 0.01).The results of this research are particularly important for identifying individual differences in 3D thinking skills of secondary school students and creating personalized dynamic intelligent learning environments.