The South Side Railroad of Long Island was a railroad company in the U.S. state of New York. Chartered in 1860 and first opened in 1867 as a competitor to the Long Island Rail Road, it was reorganized in 1874 as the Southern Railroad of Long Island and leased in 1876 to the LIRR. After a reorganization as the Brooklyn and Montauk Railroad in 1879 (immediately after which it was again leased to the LIRR) it was merged in 1889.
The main line of the South Side Railroad is now the Montauk Branch of the LIRR from Long Island City to Jamaica, the Atlantic Branch from Jamaica to Valley Stream, and the Montauk Branch again from Valley Stream to Patchogue. The Brooklyn and Montauk extended the line to Eastport while leased to the LIRR. The South Side also owned or leased lines that are now the Bushwick Branch and Far Rockaway Branch, as well as the IND Rockaway Line of the New York City Subway from Far Rockaway to Hammels (abandoned beyond Hammels to Rockaway Park) and an abandoned branch from Valley Stream to Hempstead.