Southeastern is a train operating company in the United Kingdom. It began operations in south-east England on 1 April 2006, replacing the former publicly-owned operator South Eastern Trains and serves the commuter routes to south-east London, Kent, and parts of East Sussex. The London termini of its services are Charing Cross, Blackfriars, Cannon Street, and Victoria. Southeastern operate on 773 miles (1237 km) of track, with 182 stations. 82% of its train services run into London.

It is owned by Govia, which is itself jointly owned by Go-Ahead Group and Keolis, who also operate the neighbouring Southern operating company which overlaps with Southeastern in some areas. The company’s formal name, under which it mounted its bid for the franchise, is London and South Eastern Railway. Although it continued to use the logo and livery of its predecessor for its first year of operations, a new company logo was adopted early in 2007 and stations, beginning with Waterloo East, have begun to be re-painted under the new corporate colour scheme. In August 2007, a Class 508 train was painted in a new trial livery, although the company has stated that the exact design is yet to be finalised.

The first electrostar to receive Southeastern's revised livery is No 375624. This unit was seen at Ramsgate on February 10th, and pictured in The Railway Magazine April 2008 issue.