A custom-built or homebuilt computer is a computer assembled from available components, usually commercial off-the-shelf (COTS) components, rather than purchased as a complete system from a computer system supplier, also known as pre-built systems.

A custom-built or homebuilt computer is usually considered cheaper to assemble as compared to buying a pre-built computer, since it excludes the labour cost associated with building the computer. However, depending on the person's budget, build quality, and total cost of parts used, it can still be expensive to build a custom-built computer or equivalent (see Costs of building computers).

Homebuilt computers are almost always used at home, like home computers, but home computers are traditionally purchased already assembled by the manufacturer. However, some suppliers provide both home and homebuilt computers, like the Newbear 77-68, which the owner was expected to assemble and use in his or her home.