Mechanized infantry are infantry units equipped with armored personnel carriers (APCs) or infantry fighting vehicles (IFVs) for transport and combat (see also armoured corps).

As defined by the United States Army, mechanized infantry is distinguished from motorized infantry in that its vehicles provide a degree of armor protection and armament for use in combat, whereas motorized infantry are provided with "soft-skinned" wheeled vehicles for transportation only. Most APCs and IFVs are fully tracked or are all-wheel drive vehicles (6×6 or 8×8), for mobility across rough ground. Some militaries distinguish between mechanized and armored (or armoured) infantry, designating troops carried by APCs as mechanized and those in IFVs as armored.

The support weapons for mechanized infantry are also provided with motorized transport, or they are built directly into combat vehicles to keep pace with the mechanized infantry in combat. For units equipped with most types of APC or any type of IFV, fire support weapons, such as machine guns, autocannons, small-bore direct-fire howitzers, and anti-tank guided missiles are often mounted directly on the infantry's own transport vehicles.

Compared with "light" truck-mobile infantry, mechanized infantry can maintain rapid tactical movement and, if mounted in IFVs, have more integral firepower. They require more combat supplies (ammunition and especially fuel) and ordnance supplies (spare vehicle components), and a comparatively larger proportion of manpower is required to crew and maintain the vehicles. For example, most APCs mount a section of seven or eight infantrymen but have a crew of two. Most IFVs carry only six or seven infantry but require a crew of three. To be effective in the field, mechanized units also require many mechanics, with specialized maintenance and recovery vehicles and equipment.

As early as 1915 the British instigated a tracked vehicle that could carry 50 equipped troops under armour but the project got no further than trials before cancellation. Some of the first mechanized infantry were German assault teams mounted on A7V tanks during World War I. The vehicles were extra-large to let them carry sizeable assault teams and would regularly carry infantry on board in addition to their already large crews that were trained as stormtroopers. All machine-gun-armed A7V tanks carried two small flamethrowers for their dismounts to use. A7V tank would often carry a second officer to lead the assault team.

During the Battle of St. Quentin in late March 1918, A7Vs were accompanied by twenty stormtroopers from Rohr Assault Battalion, but it is unspecified if they were acting as dismounts or were accompanying the tanks on foot. During the battle, tank crews were reported to have dismounted and attacked enemy positions with grenades and flamethrowers on numerous occasions.

Another example is the capture of Villers-Bretonneux, in which A7Vs suppressed the defenders with machine gun fire and assault teams dismounted to attack them with grenades.

The British heavy tank design was given an extended hull to cross wide German trenches. This Mark V** had space for fourteen troops. The Mark IX tank based on the Mark V was designed solely for carrying troops with space for 30 but the war ended before the order was complete and they could be used.