This study aimed to develop an assessment tool for determining the geographic inquiry process skills of 9th-12th grade high school students. A 36-item questionnaire was prepared to achieve this goal. The questionnaire was administered to 599 10th and 11th-grade students from five different school types in Ankara province of Turkey in 2019. In consequence of the exploratory and confirmatory analyses, the scale consisted of 22 items and 5 factors. These factors were named Posing Geographic Questions, Acquiring Geographic Information, Organising and Analysing Geographic Information, Answering Geographic Questions, and Communicating Geographic Information. The findings show that the scale is valid, reliable, and discriminative.