The West Coastway line is a railway line in England linking the conurbations of Brighton/Hove/Littlehampton and Southampton/Portsmouth. It has short southward branches to Littlehampton and Bognor Regis, which offer direct services to and from London.

From Brighton the East Coastway line continues to Hastings via Lewes, Eastbourne and Bexhill, and then to Ashford International, Kent via the Marshlink line.

The section east of Portsmouth was electrified by the Southern Railway before the Second World War. The electrification of the tracks further west in the late 1980s enabled electric trains to travel the whole route via Netley to Southampton or via Botley to Eastleigh. The London and South Western Railway ran the tracks west of Farlington Junction, north of Portsmouth, by the inland shore of Langstone Harbour. This section was served and timetabled separately before its electrification, a vestige of having had a different original railway company.

The West Coastway line runs almost alongside or within a few miles of the south coast of Sussex and Hampshire, between Brighton and Southampton.

East of Portsmouth the line was electrified (using 750 V DC third rail) by the Southern Railway before the Second World War in two stages:

Southern is the main operator of passenger services and stations on the line east of Portsmouth. Service patterns have varied over the years, but have always included a slow service (calling at most or all stations) from Brighton to Portsmouth. Regular services run from London Victoria via Gatwick which avoid Brighton by using the tunnel between Preston Park and Hove. These services run to Littlehampton (2016); similar services extended to Chichester, Portsmouth and Southampton particularly in the initial decades of open competition between operators, making use of statutory and negotiated running rights. Other services run from London Victoria via Gatwick and the Arun Valley line to Bognor Regis or along more of the West Coastway line, between Ford and nearby Chichester, to Portsmouth or Southampton. All of the Southern services are operated by electric multiple-units. Many of these trains join/"couple" or divide/"uncouple" during their journey, historically at Barnham, today at Horsham (except on Sundays, when trains still divide at Barnham).

The section west of Portsmouth sees trains from typically three operators. Its stations are managed by South Western Railway, who operate regular services from Portsmouth either to Southampton or to London Waterloo (less directly than its Portsmouth Direct Line, instead via Fareham, Botley, Hedge End and Eastleigh). Great Western Railway operate diesel passenger trains from Portsmouth to Bristol Temple Meads and Cardiff Central, with occasional services to the West Country or Great Malvern. Southern operate hourly services: between Brighton and Southampton Central; and between London Victoria and Southampton. The Southampton to London Victoria trains introduced at electrification created many direct routes, from Southampton, Swanwick, Fareham and Cosham to the West Sussex coast and particularly to Gatwick Airport.

In 2024, Southern proposed significant service pattern changes, by diverting all the services from Southampton Central to London Victoria to Portsmouth Harbour, as well as doubling Southampton Central to Brighton services, with an additional stop at Woolston. The lightly-used Littlehampton to Portsmouth & Southsea and Bognor Regis services will be replaced by a new Brighton to Chichester (via Littlehampton) service. The new changes commenced in June 2024.