State Route 521 (SR 521) is an east–west state highway in central Ohio. Existing entirely within Delaware County, the western terminus of State Route 521 is at the U.S. Route 36/State Route 37 duplex in Delaware. Its eastern terminus is at State Route 656 in the hamlet of Olive Green, located approximately 5 miles (8.0 km) north of Sunbury.

The Ohio Department of Transportation defines State Route 521 as beginning at the intersection of State Route 37/Central Avenue and Sandusky Street in downtown Delaware, following Sandusky Street south two blocks to U.S. Route 36/William Street, and then following U.S. Route 36 easterly through Delaware, past the partial interchange with U.S. Route 23, across the Olentangy River amidst the brief duplex U.S. Route 36 has with U.S. Route 42, and beyond the point where State Route 37 converges with the U.S. highway to the signalized intersection on the east side of the city where State Route 521 and the U.S. Route 36/State Route 37 duplex meet. This 1.76 miles (2.83 km) stretch of State Route 521 is not explicitly signed in the field.

The aforementioned signalized intersection with U.S. Route 36/State Route 37 on the east side of Delaware marks the start of the signed portion of State Route 521. The state highway departs the county seat of Delaware County heading northeast into Brown Township amidst a vastness of open farmland and the occasional home, passing underneath the CSX railway prior to passing T-intersections with Harris Road and Skinner Road. At the County Home Road intersection, State Route 521 turns east, and enters the hamlet of Kilbourne, where it passes amidst some homes and businesses, as well as intersects Old State Road. East of there, State Route 521 passes amidst woodlands as it crosses over Alum Creek and passes its intersection with Hogback Road. As it re-enters primarily open farmland, State Route 521 passes Kilbourne Road, then follows an S-curve to the southeast. Resuming an easterly trek, after passing the unusually-named 3 Bs & K Road, State Route 521 crosses into Kingston Township. Next, the state highway crosses Galena Road, then passes over Interstate 71 without much fanfare. Continuing east through farm country and passing the occasional house, State Route 521 passes through intersections with Carter's Corners Road and Blue Church Road, then turns to the east-southeast prior to entering Porter Township and arriving at its junction with State Route 61. Heading due from State Route 61, State Route 521 passes amidst a forested landscape, and crosses the Big Walnut Creek prior to coming to an end in the hamlet of Olive Green at the State Route 656 intersection. Continuing to the east of State Route 656 after State Route 521 terminates is Olive Green Road.

State Route 521 is not included within the National Highway System, a system of highways that are important to the nation's economy, defense and mobility.

State Route 521 was established in 1937 along the routing that it occupies to this day, and has not seen any significant changes since its inception.

The entire route is in Delaware County.