Pennsylvania Route 340 (PA 340) is a 30-mile (48 km) long state highway located in Lancaster and Chester counties in Pennsylvania. The western terminus is at Pennsylvania Route 462 in Lancaster. The eastern terminus is at U.S. Route 30 Business in Thorndale.

PA 340 runs parallel to US 30 for most of its length, and serves as an alternate route that bypasses much of the development along US 30. Several villages exist along the route, including, from west to east: Intercourse, White Horse, Compass, Wagontown, and Thorndale.

PA 340 was assigned in 1928 to an alignment extending from US 222 east of Lancaster to US 30 north of Gap via White Horse. In 1936, PA 340 was extended northwestward to US 22 (Herr Street) east of Harrisburg and eastward from White Horse to US 30 in Coatesville. The former alignment of PA 340 between White Horse and Gap became part of an extended PA 897. PA 340 was extended eastward to Thorndale in 1946 and truncated to US 422 near Hershey in 1955. It was truncated three more times during the 1960s, eventually moving the western end of the route to its current location. The first of these came in 1961 when the route was cut back to PA 441 east of Marietta. In 1967, it was truncated and slightly realigned to follow Prospect Road to a new terminus at US 30 west of Mountville. PA 340 was reduced to its current length in 1970.

The former routing of PA 340 in Dauphin County between US 22 near Harrisburg and Nyes Road in Lower Paxton Township became State Route 3020 (SR 3020), an unsigned quadrant route, in 1987.[citation needed] The highway, named Canby Street and Union Deposit Road, is 5.27 miles (8.48 km) in length. From Nyes Road to Union Deposit, old PA 340 is now SR 2010. Farther southeast, the former alignment of PA 340 from Hershey to Marietta is now part of PA 743 while the section from Marietta to Lancaster is part of PA 23.

Several of the buildings along this route were established under British rule, and were built as early as 1714 as part of the King's Highway(see Compass Inn).