The British Rail Class 73 is a British electro-diesel locomotive. The type is unusual in that it can operate from the Southern Region's 650/750 V DC third-rail or an on-board diesel engine to allow it to operate on non-electrified routes. This makes it very versatile, although the diesel engine produces less power than is available from the third-rail supply so the locomotives are rarely operated outside of the former Southern Region of British Rail.

Following the withdrawal and scrapping of the more powerful Class 74 electro-diesels in 1977, the Class 73 was unique on the British railway network until the introduction of the Class 88 electro-diesels in 2017. Ten locomotives have been scrapped. Since privatisation, the Class 73 fleet has been reduced in size following the large-scale withdrawals of the EWS and Gatwick Express fleets. However, many smaller operators have acquired locomotives, so their continued use is assured for the foreseeable future.