State Route 503 (SR 503) is a 54.11-mile-long (87.08 km) state highway serving Clark and Cowlitz counties in the U.S. state of Washington. The highway travels north from a short concurrency with SR 500 in Orchards through Battle Ground, the eastern terminus of SR 502, and communities in rural Clark County before crossing the Lewis River on the Yale Bridge. SR 503 intersects its spur route and turns west to parallel the Lewis River downstream to Woodland, where the highway ends at an interchange with Interstate 5 (I-5). The highway was part of the Lewis River Road, signed as State Road 15, from 1909 until 1919. The current route of SR 503 was split between Secondary State Highway 1S (SSH 1) from Woodland to Battle Ground and SSH 1U from Battle Ground to Orchards in 1937, combined to form SR 503 during the 1964 highway renumbering. A spur route, established in 1991, travels northeast into the Gifford Pinchot National Forest along Yale Lake, serving the community of Cougar.

Clark County is the southernmost county in the U.S. state of Washington. As of the 2020 census, the population was 503,311, making it Washington's fifth-most populous county. Its county seat and largest city is Vancouver. It was the first county in Washington, first named Vancouver County in 1845 before being renamed for William Clark of the Lewis and Clark Expedition in 1849. It was created by the Provisional Government of Oregon in Oregon Country on August 20, 1845, and at that time covered the entire present-day state. Clark County is the third-most-populous county in the Portland-Vancouver-Hillsboro Metropolitan Statistical Area, and is across the Columbia River from Portland, Oregon.