Harecastle Tunnel is a canal tunnel on the Trent and Mersey Canal in Staffordshire between Kidsgrove and Tunstall. The tunnel, which is 1.6 mi (2.6 km) long, was once one of the longest in the country. Its industrial purpose was for the transport of coal to the kilns in the Staffordshire Potteries. The canal runs under the 195 m (640 ft) Harecastle Hill near Goldenhill, the highest district in Stoke-on-Trent.

Although described singularly as a tunnel, Harecastle is actually two separate but parallel tunnels built almost 50 years apart. The first was constructed by James Brindley in the late 18th century and the second larger tunnel was designed by Thomas Telford, and opened in the late 1820s.

Only the Telford tunnel remains navigable after a partial collapse closed the Brindley tunnel shortly before the First World War. As the Telford tunnel is only wide enough for a single boat, canal traffic is managed by sending alternating northbound and southbound groups of boats through the tunnel. Ventilation is provided by large electric fans at the south portal.

The first tunnel through Harecastle Hill was designed by canal engineer, James Brindley. Construction began in 1770 when the surveyed route of the tunnel was marked over the hill. Fifteen vertical shafts were then sunk into the ground from which navvies mined outwards from the bottom of the shafts to create the canal line. However, changes in rock type which ranged from soft earth to Millstone Grit caused engineering problems.

The tunnel sites also flooded regularly until Watt steam engines were introduced to operate pumps. Stoves were installed at the bottom of upcast pipes to overcome the problem of ventilation. Despite the death of Brindley in 1772, the first tunnel – which measured 2,880 yd (2,630 m) long – was completed in 1777. On opening, it overtook Norwood Tunnel on the Chesterfield Canal (also bored by Brindley) as the longest tunnel on Britain's canal network.

As the tunnel had no towpath, boatsmen had to leg their way through the tunnel. Legging was done by lying on the roof of a boat and using the feet to push forward against the tunnel walls. It was slow hard work. Travel times through the tunnel averaged three hours. While the narrowboats went via the tunnel, boat horses were led over Harecastle Hill via "Boathorse Road". A lodgekeeper (now Bourne Cottage at 53°4′57.26″N 2°14′35.75″W / 53.0825722°N 2.2432639°W / 53.0825722; -2.2432639) monitored the movement of the tow-horses, who were often led by boat children, as they crossed the high ground between Kidsgrove and Tunstall.

Within years of the Brindley tunnel opening, its limitation in design soon became evident. The Industrial Revolution had resulted in rapid growth and increased demand for coal and other raw materials in the Potteries. However, as the canal tunnel was only 12 ft (3.7 m) high at its tallest point and had a maximum width of 9 ft (2.7 m), its limited capacity had become a major problem. In the early 19th century, it was decided that a second tunnel should be built by Thomas Telford. Brindley tunnel was used for the rest of the 19th century until it began to suffer an increase in subsidence in the early 20th century. In 1914 it was closed permanently after a partial collapse.

Regular engineering inspections of the disused Brindley tunnel ceased in the 1960s. Since then, no further exploration of the interior has been made beyond any significant distance from the north or south portals. Both entrances are gated and are no longer reachable by boat. Water entering the canal from the Brindley tunnel has been blamed for much of the prominent iron ore leaching into the canal (responsible for the rusty colour of the water). A reed bed has been installed at the south portal to filter the water.