A custom car is a passenger vehicle that has been altered to improve its performance, change its aesthetics, or both. Some automotive enthusiasts in the United States want 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 usage, according to Collins English Dictionary, is built to the buyer's own specifications.

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

Some of the earliest examples of modified cars were cars modified for racing or off-roading. The coachbuilding industry is considered part of custom car history, as companies and individuals built custom bodies to be fitted to early cars and inspired later customizers.

Hot rods were an early type of custom car first popularized in the United States, considered to be one of the earliest defined car customization movements. The origins of the first hot rods are typically considered to be early race cars built to race on dirt tracks and dry lake beds, often stripped down Ford Model Ts, Model As, and other pre-World War II cars made into speedsters and "gow jobs". The "gow job" morphed into the hot rod in the 1940s to 1950s. The modified cars used in the Prohibition era by bootleggers to evade revenue agents and other law enforcement are also considered a predecessor to the hot rod. Hot rods gained popularity after World War II, particularly in California, because many returning soldiers had received technical training. Many cars were "hopped up" with engine modifications such as adding additional carburetors, high compression heads, and dual exhausts. The suspension was often altered, and engine swaps were to install the most powerful engine in the lightest possible frame and body combination.

Another example of early automobile customization were the first off-road vehicles. Some of the earliest dedicated offroad vehicles were made using the Kégresse track system, starting in the late 1910s, which affixed tracks to an ordinary car in place of the rear wheels for improved off-road traction. After World War II, the surplus of army Jeeps led to a growth in the popularity of off-roading as a hobby.

Starting in the early 1940s, some US car customizers began to modify cars with a stronger emphasis on looks and self-expression. This led to styles of modification such as lowriders, kustoms and lead sleds emerging and growing. 1950s kustom car builders would often swap trim and panels from other cars, cut through the sheet metal and remove bits to make the car lower, weld it back together, and add lead to make the resulting form smooth. They would also chop the roof to make it lower, section the body to make it thinner from top to bottom, and channel the body by cutting notches in the floorpan where the body touches the frame to lower the whole body.

The first drag strip in the United States opened in 1950 on an airfield in Southern California, and a year later, in 1951 the National Hot Rod Association was formed. In the following years, more drag strips were built across the country, leading to a rise in the popularity of drag racing among both amateurs and professionals.

In the post-World War II era, Japan's automotive industry grew, eventually leading to the country becoming the world's largest vehicle producer. This led to a unique car customization culture within the nation. Some of the early custom cars in Japan, starting in the late 1970s through the 1980s, included Kaido Racers, Japanese cars modified with homemade parts to look like racecars of the time; imported and modified American and European cars; cars modified for top-speed and highway racing; and Dekotora decorated trucks. The 1990s saw the rise of cars modified for drifting, VIP style luxury sedans, and the continued popularity of highway racing. Japan also embraced American customization styles, importing and building their muscle cars, lowriders, minitrucks, and more.