A turnkey, a turnkey project, or a turnkey operation (also spelled turn-key) is a type of project that is constructed so that it can be sold to any buyer as a completed product. This is contrasted with build to order, where the constructor builds an item to the buyer's exact specifications, or when an incomplete product is sold with the assumption that the buyer would complete it.

A turnkey project or contract as described by Duncan Wallace (1984) is:

…. a contract where the essential design emanates from, or is supplied by, the Contractor and not the owner, so that the legal responsibility for the design, suitability and performance of the work after completion will be made to rest … with the contractor …. 'Turnkey' is treated as merely signifying the design responsibility as the contractor's.

Turnkey contract is typically a construction contract under which a contractor is employed to plan, design and build a project or an infrastructure and do any other necessary development to make it functional or ‘ready to use’ at an agreed price and by a fixed date.

In Turnkey contracts, most of the time employer provides the primary design. The contractor must follow the primary design provided by the employer.

A turnkey computer system is a complete computer including hardware, operating system and application(s) designed and sold to satisfy specific business requirements.