Pennsylvania Route 999 is a 9.44-mile (15.19 km) long, east–west state highway located in western Lancaster County, Pennsylvania. The western terminus is at PA 441, which has its southern terminus at this intersection, in Washington Boro. The eastern terminus is at PA 462 in Lancaster. It is the highest numbered state road in Pennsylvania.

PA 999 has many names along the route. The names include Penn Street, Blue Rock Road, Manor Avenue, and Millersville Pike. Most locals however, will refer to the route as Millersville Pike throughout the entire route.

The route begins in the village of Washington Boro, at the intersection of the southern terminus of PA 441, and River Road. The route heads east to the town of Millersville. After crossing through the town, it heads northeast towards the city of Lancaster. The route terminates in the city at PA 462 eastbound (King Street). The route does not connect PA 462 westbound however, and to get to it, one must drive up 2 blocks up King Street to Mulberry Street, and go up one block to get to westbound PA 462 (Orange Street)

The portion east of Millersville was once the Lancaster and Millersville Turnpike, a private toll road. The route was signed in 1928. In 1932, PA 999 was paved from Washington Boro to Millersville. The route has stayed on the same roads since its inception.

The entire route is in Lancaster County.