SR 821 begins at an interchange with I-82, co-signed with US 97, northeast of Selah in northern Yakima County. The interchange also includes a connection to Firing Center Road, which continues east into the Yakima Training Center, a military installation that spans the mountainous area between Yakima and the Columbia River. SR 821 makes a 90-degree turn to the northeast and intersects the northern terminus of SR 823, a short highway that travels south into downtown Selah. The highway continues north through farms near the community of Pomona and passes the Sundown M Ranch substance abuse treatment center near the mouth of Selah Creek at the Yakima River. It then travels into the narrow Yakima River Canyon to the east of BNSF Railway's Yakima Valley Subdivision, which forms part of the main east–west railroad across south central Washington.