The Williamstown line is a commuter railway line in the city of Melbourne, Victoria, Australia. Operated by Metro Trains Melbourne, it is the city's third shortest metropolitan railway line at 16.2 kilometres (10.1 mi).

The line runs from Flinders Street station in central Melbourne to Williamstown station in the inner west, serving 12 stations via Footscray, Yarraville, and Newport. The line operates for approximately 19 hours a day (from approximately 5:00 am to around 12:00 am) with 24-hour service available on Friday and Saturday nights. During peak hours, the line operates with headways of up to 20 minutes, ensuring frequent service for commuters. During off-peak hours, the service intervals are adjusted to provide service every 20–30 minutes, accommodating the lower demand. Trains on the Williamstown Line run with two three-car formations of Comeng, Siemens Nexas, and X'Trapolis 100 trainsets.

Together with the Southern Cross to Sunbury portion of the Deniliquin railway line, it is the first and oldest government-built line in Victoria, having opened on 13 January 1859. The line was built to serve the port at Williamstown, giving it economic importance to the then-colony as it established itself as an important international passenger port. The line has played an important part in the development of Victoria's railways, with the first workshops opening along the line in the 1800s.

Since the 2010s, due to the heavily utilised infrastructure of the Williamstown line, improvements and upgrades have been made. Works have included replacing sleepers, upgrading signalling technology, the removal of level crossings, the introduction of new rolling stock, and station accessibility upgrades.

Construction of the line was started by the Melbourne, Mount Alexander, and Murray River Railway Company (MMA&MRR) with the construction of Batman Hill (now Southern Cross) station in 1853. Interestingly, the station did not see a train for another 6 years till services began in 1857. In response to 12 months of inaction by the MMA&MRR and pressure from the government and financial backers, the company organised a gala event at Williamstown on 12 June 1854.

Recognising the challenges faced by private companies and the pressing need for railway development, the government took action to address the situation. On 19 March 1856, despite only small sections being completed, the government intervened and acquired the line, assuming responsibility for its construction and further development.

In 1858, the Williamstown Workshops opened for the assembly of engines and carriages imported from England. Other buildings were soon added, with a total of seven locomotives built. During the late 1880s when railway management described the workshops as inadequate, moves were made to construct new workshops at Newport. By 1889 the new shops were open and Williamstown was closed.

Although it is now operated as a branch from the main Werribee/Geelong line at Newport, the line was originally built from the city with the Geelong line being the branch. The line officially opened in January 1859, but the section between the Newport Workshops and Williamstown Pier was in use by Geelong-line trains from October 1857 as it provided a connection to the ferry services departing from the pier. From Williamstown Pier, passengers could connect to a ferry across Hobsons Bay to Port Melbourne.