Mega City or megacity may refer to:

• Megacity is a recognized metropolitan area with a total population in excess of 10 million people.
• Mega City (The Matrix), or simply "The City", is the virtual city created within the reality of the Matrix series
• Mega-City One and Mega-City Two, from the Judge Dredd comic book series
• Mega City Four, a British pop-grunge/crusty band from the late 1980s
• Mega City Comics (bookshop), comic book store based in Camden Town, London
• In Canada, the term often refers to the merger of municipalities and their governments.

A megacity is usually defined as a metropolitan area with a total population in excess of ten million people. A megacity can be a single metropolitan area or two or more metropolitan areas that converge. The terms conurbation, metropolis and metroplex are also applied to the latter.