A V6 engine is a six-cylinder piston engine where the cylinders and cylinder blocks share a common crankshaft and are arranged in a V configuration.

The first V6 engines were designed and produced independently by Marmon Motor Car Company, Deutz Gasmotoren Fabrik and Delahaye. Engines built after World War II include the Lancia V6 engine in 1950 for the Lancia Aurelia, and the Buick V6 engine in 1962 for the Buick Special. The V6 layout has become the most common layout for six-cylinder automotive engines.

Due to their short length, V6 engines are often used as the larger engine option for vehicles which are otherwise produced with inline-four engines, especially in transverse engine vehicles. A downside for luxury cars is that V6 engines produce more vibrations than straight-six engines. Some sports cars like the Porsche 911 use flat-six engines instead of V6 engines, due to their near perfect primary engine balance and lower centre of gravity (which improves the handling).

The displacement of modern V6 engines is typically between 2.5 and 4.0 L (153 and 244 cu in), though larger and smaller examples have been produced, such as the 1.8 L (110 cu in) Mazda V6 used in the 1991–1998 Mazda MX-3, or the 1.6 L (98 cu in) Mitsubishi V6 engine used in the 1992–1998 Mirage/Lancer, while the largest gasoline V6 built was the 7.8 L (478 cu in) GMC V6 used in the 1962 GMC C/K series 6500.

All V6 engines with even firing spacing—regardless of the V-angle between the cylinder banks—are subject to a primary imbalance caused by each bank consisting of an inline-three engine, due to the odd number of cylinders in each bank. Straight-six engines and flat-six engines do not experience this imbalance. To reduce the vibrations caused by this imbalance, most V6 engines use a harmonic damper on the crankshaft and/or a counter-rotating balance shaft.

Six-cylinder designs have less pulsation in the power delivery than four-cylinder engines, due to the overlap in the power strokes of the six-cylinder engine. In a four-cylinder, four-stroke engine, only one piston is on a power stroke at any given time. Each piston comes to a complete stop and reverses direction before the next one starts its power stroke, which results in a gap between power strokes, especially at lower engine speeds (RPM). In a six-cylinder engine with an even firing interval, the next piston starts its power stroke 60° before the previous one finishes, which results in smoother delivery of power to the flywheel.

Comparing engines on a dynamometer, a V6 engine shows instantaneous torque peaks of 154% above mean torque and valleys of 139% below mean torque, with a small amount of negative torque (engine torque reversals) between power strokes. In the case of a four-cylinder engine, the peaks are approximately 270% above mean torque and 210% below mean torque, with 100% negative torque being delivered between strokes. However, a V6 with uneven firing intervals of 90° and 150° shows large torque variations of 185% above and 172% below mean torque.

Since 1991, Volkswagen has produced narrow angle VR6 engines with V-angles of 10.5 and 15 degrees shared by both banks of cylinders, in a design similar to the 1922-1976 Lancia V4 engine. These engines use a single cylinder head so are technically a straight engine with the name "VR6" coming from the combination of German words “Verkürzt” and “Reihenmotor” meaning “shortened inline engine”. The VR6 engines were used in transverse engine front-wheel drive cars which were originally designed for inline-four engines. Due to the minimal extra length and width of the VR6 engine, it could be fitted to the engine compartments relatively easily, in order to provide a displacement increase of 60 percent .