This study aimed to investigate the impact of inquiry-based indoor and outdoor mathematics activities on pre-school children's geometric and spatial thinking skills. Thirty-two children between 48 and 66 months old participated in the study. The study employed as a quasi-experimental research model and employed Non-equivalent Control Group Design. The design included one control group and two experimental groups. The control group continued regular curriculum activities while Experimental Group 1 was exposed to inquiry-based mathematics activities in the classroom, and Experimental Group 2 engaged in the same activities but outdoors, along with the regular curriculum activities. The Geometric and Spatial Thinking Skills Test was used as a data collection tool. According to the results of the study, inquiry-based mathematics activities contribute to preschoolers' geometric and spatial thinking skills. The impact of inquiry-based mathematics activities on experimental groups were similar to each other. Some recommendations for further studies and implementations were made following the results of this study.