The Portland Branch railway refers to a group of lines on the Isle of Portland in the English county of Dorset. The first was the Portland Railway, a tramway with a counterbalanced rope-worked incline. It opened in 1826. It was followed by the Weymouth and Portland Railway, which connected to the main line of the Great Western Railway at Weymouth. It opened in 1865. From the late 1840s until 1872, Portland Breakwater was built, a prodigious construction task that created a very large safe harbour. It was decided to provide a railway connection to the breakwater, which was used as a pier for bunkering ships. This was constructed by the LSWR and the GWR jointly and opened in 1876. The fourth line was the Easton and Church Hope Railway. This line was conceived as a simple descent to bring stone down from quarries to a new jetty at Church Ope, but after their line was authorised in 1867, the Company delayed useful construction, and a change of plan followed, with several Acts of Parliament authorising modifications to the route and extension of time. It finally opened in 1900.

The Isle of Portland is a tied island, 6 kilometres (4 mi) long by 2.7 kilometres (1.7 mi) wide, in the English Channel. The southern tip, Portland Bill, lies 8 kilometres (5 mi) south of the resort of Weymouth, forming the southernmost point of the county of Dorset, England. A barrier beach called Chesil Beach joins Portland with mainland England. The A354 road passes down the Portland end of the beach and then over the Fleet Lagoon by bridge to the mainland. The population of Portland is 13,417.