A double-decker bus or double-deck bus is a bus that has two storeys or decks. Double-deckers are primarily for commuter transport, but open-top models are used as sight-seeing buses for tourists, and there are coaches too 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 and Ireland, in Hong Kong, and in Singapore.

A bus is a road vehicle that carries significantly more passengers than an average car or van, but less than the average rail transport. It is most commonly used in public transport, but is also in use for charter purposes, or through private ownership. Although the average bus carries between 30 and 100 passengers, some buses have a capacity of up to 300 passengers. The most common type is the single-deck rigid bus, with double-decker and articulated buses carrying larger loads, and midibuses and minibuses carrying smaller loads. Coaches are used for longer-distance services. Many types of buses, such as city transit buses and inter-city coaches, charge a fare. Other types, such as elementary or secondary school buses or shuttle buses within a post-secondary education campus, are free. In many jurisdictions, bus drivers require a special large vehicle licence above and beyond a regular driving licence.