It happened at the Slavyansky Bulvar metro station in the west of the city.
15 people were also injured in the crash, police told local media.
Cops are at the scene and the driver has reportedly been detained.
INSTAGRAM
Horrifying footage shows the bus driving down the passage as helpless pedestrians are crushed underneath.
Other CCTV shows the vehicle slowly approaching the station after navigating past parked cars.
Three medical helicopters have been dispatched and the Mayor of Moscow is reportedly at the scene.
The bus’ brakes may have failed or it may have been affected by a car that cut in front of it, police believe.
Bus ploughs into crowd at Moscow underground station

На западе Москвы автобус протаранил группу людей, есть погибшие https://t.co/c2qHL840fn pic.twitter.com/DXM3fzgDOX
— Ruposters (@Ruposters) December 25, 2017
MORE TO FOLLOW