A custom car is a passenger vehicle that has been either substantially altered to improve its performance, often by altering or replacing the engine and transmission; made into a personal "styling" statement, using paint work and aftermarket accessories to make the car look unlike any car as delivered from the factory; or some combination of both. A desire among some automotive enthusiasts in the United States is to push "styling and performance a step beyond the showroom floor - to truly craft an automobile of one's own." A custom car in British according to Collins English Dictionary is built to the buyer's own specifications.

Although the two are related, custom cars are distinct from hot rods. The extent of this difference has been the subject of debate among customizers and rodders for decades. Additionally, a street rod can be considered a custom.

Custom cars are not to be confused with coachbuilt automobiles, historically rolling chassis fitted with luxury bodywork by specialty body builders.