The Frankston line is a commuter railway line in the city of Melbourne, Victoria, Australia. Operated by Metro Trains Melbourne, it is the city's third-longest metropolitan railway line, at 42.7 kilometres (26.5 mi). The line runs from Flinders Street station in central Melbourne to Frankston station in the south-east, serving 28 stations via South Yarra, Caulfield, Moorabbin, and Mordialloc. The line continues to Stony Point on the non-electrified Stony Point line. The line operates for approximately 20 hours a day (from approximately 4:00 am to around 11:30 pm) with 24 hour service available on Friday and Saturday nights. During peak hour, headways of up to 5 to 10 minutes are operated with services every 10–20 minutes during off-peak hours. Trains on the Frankston line run with a two three-car formations of Comeng, Siemens Nexas, and X'Trapolis 100 trainsets.

Sections of the Frankston line opened as early as 1881, with the line fully extended to Frankston in 1882. A limited number of stations were first opened, with infill stations progressively opened between 1881 and 2017. The line was built to connect Melbourne with the rural towns of Caulfield, Moorabbin, and Frankston, amongst others. Significant growth has occurred since opening, with a plan to extend the Frankston line along part of the Stony Point line to Baxter.

Since the 2010s, due to the heavily utilised infrastructure of the Frankston line, significant improvements and upgrades have been made. Different packages of works have upgraded the corridor to replace sleepers, upgrading signalling technology, the introduction of new rolling stock, and the removal of all level crossings.

In 1881, the Frankston line began operations by the Victorian Railways on a single tracked line from Caulfield to Mordialloc. Mordialloc station was officially opened on 19 December 1881 by Sir Thomas Bent, a corrupt politician who was the Minister of Railways, and later becoming the Premier of Victoria between 1904 and 1909 and had purchased land in Melbourne’s south east to build a train line toward it and then re-sell the land for significant profit. The first train to arrive at Mordialloc was a special service from Princes Bridge, which collected school children from the Brighton area. Further specials occurred during the day, with proper timetabled services commencing the following day. Six services were provided upon opening and, apart from two services, all were shuttle services operating between Caulfield and Mordialloc.

In August 1882, operations were extended from Mordialloc to Frankston. The section of track from Caulfield to Mordialloc was duplicated in 1888. In 1883 the line between Richmond station and South Yarra was quadrupled to accommodate an increase in train services due to the opening of Frankston and Sandringham lines.

In 1885, a number of level crossing removal works occurred between Flinders Street station and South Yarra due to an increase in freight and passenger operations. These crossings were removed through a combination of lowering and raising the corridor.

In 1910, the remaining track from Mordialloc to Frankston was duplicated. In 1915, the line between South Yarra and Caulfield was quadrupled, as part of level crossing removal works. This section of the line was lowered into a cutting to eliminate numerous level crossings. Power signalling was provided between Richmond and Hawksburn at the same time, then on to Caulfield in 1921.

Electrification of the line to Frankston occurred in three stages during 1922. In March 1922, the section from South Yarra to Glen Huntly station was electrified, with the section to Mordialloc being electrified in June 1922, and the final section to Frankston being completed by August 1922. The electrification of the line allowed for the introduction of Swing Door electric multiple unit trains for the first time.