Monument is located in the northern portion of Grant County in eastern Oregon. The city sits in the valley of the North Fork John Day River, a major tributary known for its free-flowing waters and ecological importance to regional fish populations.
The surrounding geography is dominated by the transition from the high desert of the Columbia Plateau to the forested slopes of the Blue Mountains. The landscape is a striking mix of semi-arid sagebrush steppe, grassy foothills, and pine-studded ridges. The town is largely isolated, surrounded by vast tracts of public land, including the Umatilla National Forest to the north and east.