Pennsylvania Route 851 is a 32-mile (51 km) long state highway located in York County in Pennsylvania. The western terminus is at PA 516 in Manheim Township. The eastern terminus is at PA 74 in Peach Bottom Township.

Beginning at the western terminus, PA 851 begins at the intersection of PA 516 & PA 216 in Codorus Township. From there, PA 851, known as Steltz Road, heads south toward the Maryland/Pennsylvania line. The road makes a 90 degree turn at the state line, and runs parallel to it for about 1/2 mile, then curves northeastward and crosses into Shrewsbury Township. It then makes a second 90 degree turn in the Borough of New Freedom, where it is picked up on 2nd Street. From here, it crosses into the Borough of Railroad, and bears right at the intersection with PA 616. It then proceeds into the Borough of Shrewsbury, and is picked up on East Forrest Avenue. It then underpasses Interstate 83 as the first Pennsylvania exit. PA 851 then crosses over the northern branch of Deer Creek, and crosses into Hopewell Township.

At this point, it is named Bridgeview Road. It then enters Stewartstown, where it is picked up on W. Pennsylvania Ave for about 1/4 mile. It is concurrent with PA 24 for about 1/2 mile, before being picked up on College Ave. From there, it later becomes Pleasant Valley Road. PA 851 bears right onto Woolen Mill Road, crossing into Fawn Township, where it intersects with New Park Road. It bears right onto Main Street and heads into the Borough of Fawn Grove, where it intersects PA 425. It then bears left at the intersection with Quadrant Route 2080 (Graceton Road) which becomes Maryland Route 624 at the state line and Constitution Road, and crosses into Peach Bottom Township. From there, it bears right at the Five Forks intersection with Miller Road, Hushon Road, and Aubel Road. It then continues as Bryansville Road for a few miles then terminates at its intersection with PA 74.

The entire route is in York County.

Pennsylvania Route 851 Truck is a 3-mile (4.8 km) long truck route between New Freedom and Shrewsbury, Pennsylvania. The designation was established to avoid a narrow intersection and a level crossing in the tiny, but appropriately named Railroad, Pennsylvania. The route runs as Constitution Avenue in New Freedom and then becomes the much more important Main Street in Shrewsbury during its short run.