The Mayflower line is a railway branch line in the east of England that links Manningtree, on the Great Eastern Main Line, to Harwich Town. During peak times, many services connect to or from the main line and its London terminus at Liverpool Street. The Mayflower line has six stations, including the two termini, and is situated within the county of Essex.

The route is 11 miles 16 chains (18.02 km) in length from where it branches off the main line west of the town of Manningtree to its eastern terminus in Harwich. It is part of Network Rail Strategic Route 7, SRS 07.07, and is classified as a London and South East commuter line. The Engineer's Line Reference for the line is MAH.

As of December 2016, passenger services on the Mayflower line are operated by Greater Anglia, which also manages all of the stations. The typical service frequency is one train per hour in each direction. The timetabled journey time between Manningtree and Harwich Town is 22 minutes.

The Mayflower line takes its name from the Pilgrim ship Mayflower, which is believed to have been built in Harwich in the 16th century. Harwich was also home to the ship's captain and part-owner, Christopher Jones.

The Eastern Counties Railway (ECR) had originally proposed plans to extend what is now the Great Eastern Main Line from Colchester to Harwich, although this was a cause for concern to the town of Ipswich, which was a rival port.

In 1846 a railway line from Manningtree to Harwich proposed by the Eastern Union Railway (EUR) was approved by the Railway Commissioners. In 1853 an agreement was reached between the companies, with the ECR taking over the working of the EUR from 1 January 1854. The single-line branch opened on 15 August 1854.

In 1862 the ECR and the EUR merged to form the Great Eastern Railway (GER).

The track was doubled in 1882 by the GER, and the Manningtree North Curve which allows direct running between Ipswich and Harwich was also added at that time.