In general, many Indonesian students have difficulty learning mathematics. Mathematics is widely used and developed by humans to solve problems in everyday life. However, mathematics is rarely taught as a culturally related subject. The mosque is a cultural embodiment whose ornaments use a lot of geometric motifs. Ethnomathematics studies the relation between mathematics and culture. This research aims to explore the geometry concepts on mosque ornaments, determine how to construct mosque ornaments using geometry concepts, and create contextual geometry problems based on mosque ornaments. The research employed a qualitative approach with an ethnographic method. The research location was the Grand Mosque of Bandung, West Java, Indonesia. The researchers act as an instrument of data collection. The triangulation technique is used to generate valid data, which was carried out through observations, interviews, and literature studies. Data condensation, data display, and concluding were used to analyze the research data. This research showed that the Grand Mosque of Bandung ornaments have geometry concepts, namely points, lines, angles, triangles, squares, rhombuses, kites, trapeziums, hexagons, circles, translation, reflection, rotation, and dilation. Mosque ornaments can be constructed using geometry concepts. In addition, contextual geometry problems can be created based on mosque ornaments. These results indicate that the ornaments of the Grand Mosque of Bandung are rich in geometry concepts. Therefore, mosque ornaments can be used as alternative learning media to overcome students' mathematical difficulties, especially in geometry material.