South West Trains (SWT) is the operating name of Stagecoach South Western Trains Ltd., a British train operating company providing, under franchise, passenger rail services, mostly out of London Waterloo station, to the southwest of London in the suburbs and in the counties of Surrey, Hampshire, Dorset, Devon, Somerset, Berkshire, and Wiltshire and on the Isle of Wight. The area of operation, essentially the former South Western division of Network SouthEast, is also roughly that of the pre-1923 London and South Western Railway. The Stagecoach Group took over the franchise on the privatisation of British Rail in 1996 and retained it against stiff competition in 2007 making it, with Chiltern Railways and Virgin West Coast, one of the three longest-running franchises. It is the largest passenger franchise in the UK and is a particularly complex operation due to the large number and variety of services.

In the early days of its franchise SWT gained notoriety for severe service cuts due to driver shortages but later made significant improvements to the network, including replacing much of the rolling stock, refurbishing stations, making stations accessible to disabled passengers, and improving customer information. During the early 2000s, improvements included the introduction of new rail services and the reopening of Chandler's Ford station in Hampshire.

Southwest or south west is the ordinal direction halfway between south and west.

Southwest or south west may also refer to:

Southwestern Conference (Ohio)