The Appalachian Development Highway System (ADHS) is a series of highway corridors in the Appalachia region of the eastern United States. The routes are designed as local and regional routes for improving economic development in the historically isolated region. It was established as part of the Appalachian Regional Development Act of 1965, and has been repeatedly supplemented by various federal and state legislative and regulatory actions. The system consists of a mixture of state, U.S., and Interstate routes. The routes are formally designated as "corridors" and assigned a letter. Signage of these corridors varies from place to place, but where signed are often done so with a distinctive blue-colored sign.

The Appalachian Regional Commission (ARC) forecast benefits of ADHS' completion by FY 2045 as the creation of 47,000 new jobs and $4.2 billion in gross regional product (GRP).

In 1964, the President's Appalachian Regional Commission (PARC) reported to Congress that economic growth in Appalachia would not be possible until the region's isolation had been overcome. Because the cost of building highways through Appalachia's mountainous terrain was high, the region's local residents had never been served by adequate roads. The existing network of narrow, winding, two-lane roads, snaking through narrow stream valleys or over mountaintops, was slow to drive, unsafe, and in many places worn out. The nation's Interstate Highway System, though extensive through the region, was designed to serve cross-country traffic rather than local residents.

The PARC report and the Appalachian governors placed top priority on a modern highway system as the key to economic development. As a result, Congress authorized the construction of the Appalachian Development Highway System (ADHS) in the Appalachian Development Act of 1965. The ADHS was designed to generate economic development in previously isolated areas of the 13 Appalachian states, supplement the interstate system, and provide access to areas within the region as well as to markets in the rest of the nation.

The ADHS is currently authorized at 3,090 miles (4,970 km), including 65 miles (105 km) added in January 2004 by Public Law 108–199. A decade into construction, delays and cost increases mounted, attributed to:

Periodic ADHS Completion Plan Reports were compiled to assess construction and the remaining cost-to-complete (C-to-C) forecasts, excerpts listed below.

By FY 2023, 2,837 miles (4,566 km) – 91.8 percent of the authorized distance – were complete, open to traffic, or under construction. Many of the remaining miles will be among the most expensive to build. The ARC (the state governors) remain involved prioritizing, sequencing remaining corridor work. By 2040, 100% of ADHS' project miles are expected to be complete and open to traffic or, at least, partially complete.

Corridor Z across southern Georgia is not part of the official system, but has been assigned by the Georgia Department of Transportation.