A hemispherical combustion chamber is a combustion chamber in the cylinder head of an internal combustion engine with a domed "hemispheric" shape. An engine featuring this type of hemispherical chamber is known as a hemi engine. In practice, shapes less than a full hemisphere are typically employed, as are variations (or faceting in parts) of a true hemispheric profile. The primary advantage of such shapes are increased compression (leading to greater power) and very large intake and exhaust valves (allowing better flow of intake and exhaust gasses, also resulting in improved volumetric efficiency and greater power); the primary disadvantages are complex valve trains (caused by valves being placed opposite one-another in a head) and expense (of machining the heads and pistons, and additional valve train components).

While hemispherical combustion chambers are still found in the 2000s multi-valve arrangements (of four and even five valves per cylinder) and the popularity of overhead cam (including double overhead cam) arrangements have altered the traditional trade-offs in employing "hemi heads".

Hemispherical combustion chambers were introduced on some of the earliest automotive engines, shortly after the viability of the internal combustion engine was first demonstrated. Their name reflects the domed recess in a cylinder head and correspondingly shaped top of a piston enclosing a space that approximates a half of a sphere (hemi- + -sphere + -ical), although in practice the actual enclosed space is generally less than half.

Hemispherical cylinder heads have been used since at least 1901; they were used by the Belgian car maker Pipe in 1905 and by the 1907 Fiat 130 HP Grand Prix racer. The Peugeot Grand Prix car of 1912 and the Alfa Romeo Grand Prix car of 1914 were both four-valve engines, and Daimler and Riley were also using hemispherical combustion chambers at the time. Beginning in 1912, Stutz used four-valve engines, conceptually anticipating modern car engines. Other examples include the BMW double-pushrod design (adopted by Bristol Cars), the Peugeot 403, the Toyota T engine and Toyota V engine (Toyota's first V8 engine), Miller racing engines, and the Jaguar XK engine.

A hemispherical head ("hemi-head") gives an efficient combustion chamber with minimal heat loss to the head, and allows for two large valves. However, a hemi-head usually allows no more than two valves per cylinder due to the difficulty in arranging the valve gear for four valves at diverging angles, and these large valves are necessarily heavier than those in a multi-valve engine of similar valve area, as well as generally requiring more valve lift. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Since the combustion chamber is virtually a hemisphere, a flat-topped piston yields a lower compression ratio unless a smaller chamber is utilized.

Significant challenges in the commercialization of engines utilizing hemispherical chambers revolved around the design of the valve actuation, and how to make it effective, efficient, and reliable at an acceptable cost, which normally requires the use of either a dual rocker system, or dual camshafts to operate the inlet and exhaust valves. Complexity was referenced early in Chrysler's development of their 1950s hemi engine: the head was referred to in company advertising as the Double Rocker head. Ford's CVH (Compound Valve Hemispherical) engine of the 1980s solved the problem by way of utilizing a complex geometry of the valve angle combined with a cam-in-head configuration that allowed hemispherical arranged valves to be operated by a single camshaft and without the need for two rocker shafts.

Although a wedge-head design offers simplified valve actuation, it usually does so by placing the valves side by side within the chamber, with parallel stem axes. This can restrict the flow of the intake and exhaust into and out of the chamber by limiting the diameters of valve heads to total no more than the bore of the cylinder in a two valve per cylinder arrangement. With a hemispherical chamber with splayed valve stem angle, this limitation is increased by the angle, making the total valve diameter size possible to exceed the bore size within an overhead valve configuration. See IOE engine for another method.

Also, the splayed valve angle causes the valve seat plane to be tilted, giving a straighter flow path for the intake and exhaust to/from the port exiting the cylinder head. Engineers have learned that while increasing the valve size with straighter port is beneficial for increasing the maximum power at high rpm, it slows the intake flow speed, not providing the best combustion event for emissions, efficiency, or power in the normal rpm range.