OpenTTD is a business simulation game in which players try to earn money by transporting passengers, minerals and goods via road, rail, water and air. It is an open-source remake and expansion of the 1995 Chris Sawyer video game Transport Tycoon Deluxe.

OpenTTD duplicates most features of Transport Tycoon Deluxe and has many additions, including a range of map sizes, support for many languages, custom (user-made) artificial intelligence (AI), downloadable customisations, ports for several widely used operating systems, and a more user-friendly interface. OpenTTD also supports local area network (LAN) and Internet multiplayer, co-operative and competitive, for up to 255 players.

OpenTTD is free and open-source software licensed under the GNU GPL-2-0-only and is under ongoing development. According to a study of the 61,154 open-source projects on SourceForge in the period between 1999 and 2005, OpenTTD ranked as the 8th most active open-source project to receive patches and contributions. In 2004, development moved to their own server. Since 2018, the project uses GitHub for its source repository and bug tracker. Starting from April 1, 2021, the game is now also available on Steam.