The Alton line is a railway line in Hampshire and Surrey, England, operated by South Western Railway; it is a relatively long branch of the South West Main Line.

The branch leaves the main line at Pirbright Junction to the west of Brookwood station, Surrey, then turns to the south-west. The route crosses and recrosses the Surrey/Hampshire border, serving the towns of Aldershot and Farnham, before reaching its present-day terminus at Alton in East Hampshire. The line originally continued west to Winchester; the section between Alton and New Alresford is preserved as the heritage Watercress line.

The Alton line was electrified (750 V DC third rail) during the late inter-war years by Southern Railway. Steam trains connected to the Mid Hants Watercress Railway (by way of rolling stock supply or special excursion) operate on the line.

The Alton line is a standard-gauge railway line in South East England. It runs for 17 miles 29 chains (27.9 km) from the South West Main Line at Pirbright Junction in Surrey to Alton in Hampshire. The line is electrified using the 750 V DC third-rail system and is double track between Pirbright Junction and Farnham. Between Farnham and Alton, there is an 8 mi (13 km) single-track section, although trains may pass at Bentley station. The entire line is controlled by Woking Area Signalling Centre; the signalling system uses track circuits, with the exception of the Aldershot station area, which uses the absolute block method. The maximum permitted speed on the line is 70 mph (110 km/h) and passenger trains from Alton typically reach Brookwood on the South West Main Line in around 30–35 minutes.

At Pirbright Junction, a flyover allows trains heading from Alton towards London to pass over all four tracks of the South West Main Line. There are two additional junctions with National Rail lines: Ash Vale Junction allows trains to access the Ascot–Ash Vale line and Aldershot North Junction allows trains to access the spur towards the North Downs Line. At the southern end of the line, the connection to the Watercress Line, a heritage railway, is fully signalled.

There are two tunnels on the Alton line: the longest is the 418 yd (382 m) Foxhills Tunnel, between Pirbright Junction and Ash Vale, which runs beneath Pirbright Common; the shorter 76 yd (69 m) Aldershot Tunnel, between Ash Vale and Aldershot, passes under Redan Hill. The Alton line crosses the Basingstoke Canal twice: at Ash Vale and Aldershot Railway Bridges. The level crossing at Farnham, where the line crosses the B3001, is one of the most misused in the Network Rail Wessex Region. Between Farnham and Alton, the line parallels the north branch of the River Wey.

All five stations on the line—Ash Vale, Aldershot, Farnham, Bentley and Alton—are managed by South Western Railway (SWR), which operates all passenger services. Aldershot and Alton have three platforms, although Platform 3 at the latter is used exclusively by the Watercress Line. The other stations have two platforms each.

Three distinct passenger services run on the Alton line, all of which are operated by SWR: