The Downtown Line (DTL) is a medium-capacity Mass Rapid Transit (MRT) line in Singapore. It runs from Bukit Panjang station in the north-west of the country towards Expo station in the east via a loop around the city-centre. Coloured blue on the rail map, the line serves 34 stations, all of which are underground. The DTL is the fifth MRT line on the network to be opened and the third line to be entirely underground. It is the second MRT line to be operated by SBS Transit after the North East Line. It also serves as an alternative to the East–West Line where it runs roughly geographically parallel from Bugis to Expo station.

The Mass Rapid Transit system, locally known by the initialism MRT, is a rapid transit system in Singapore and the island country's principal mode of railway transportation. The system commenced operations in November 1987 after two decades of planning with an initial 6 km (3.7 mi) stretch consisting of five stations. The network has since grown to span the length and breadth of the country's main island – with the exception of the forested core and the rural northwestern region – in accordance with Singapore's aim of developing a comprehensive rail network as the backbone of the country's public transportation system, averaging a daily ridership of 3.4 million in 2019.