A double-decker bus is a bus that has two storeys or decks. Double-deckers are used primarily for commuter transport, but open-top models are used as sightseeing buses for tourists, and there are coach variants for long-distance travel. They appear in many places around the world, but are presently most commonly used as mass transport in cities of Britain as well as in Ireland, China, Hong Kong, Berlin and Singapore.

The earliest double-decker horse-drawn omnibus appeared in Paris in 1853 and were motorised in the 1900s. Double-decker buses were popularised in Great Britain at the start of the 20th century, with the best-known example being the red London bus, namely the AEC Routemaster. Double-deckers in urban transport were also in common use in other places, such as major cities of India, but were mostly phased out by the end of the 20th century. However, they remain common in Britain as well as Ireland and Hong Kong, while in Singapore and Dhaka they have been introduced and greatly expanded in the aftermath of British colonial rule.

There are several types of double-decker buses as shown in the imagebox below:

Early double-deckers put the driver in a separate cab. Passenger access was via an open platform at the rear and a bus conductor collected fares. Modern double-deckers have a main entrance door at the front and the driver takes fares, thus halving the number of workers aboard, but slowing the boarding process. The rear open platform, popular with passengers, was abandoned for safety reasons, as there was a risk of passengers falling when running and jumping onto the bus.

Cities listed here have double-decker buses as part of their regular mass transit fleet. Cities with only tourist and sightseeing double-decker buses are excluded.

In the European Union, the maximum height for any vehicle is 4 metres, for motor vehicles in categories M2 and M3 and their trailers in category 0 and motor vehicles in categories N2 and N3 and their trailers in categories 03 and 04, in national and international traffic according to Council directive 96/53/EC of 25 July 1996 and in continuity of council directive 85/3/CEE.

The United Kingdom has a triple standard for the double-decker bus: highbridge bus (urban Britain), lowbridge bus (countryside Britain) and 4 metres height coach such as the Neoplan Skyliner that can traverse Europe.

Outside the British Isles in Europe double-decker buses are most prominent in Skopje and Berlin.