机械化步兵是现代陆军中的一个兵种，是指搭乘步兵战车或装甲运兵车作战的步兵。

机械化步兵是在第二次世界大战期间，由德国在闪电战理论指导下开创的一个兵种。纳粹德国陆军为了加强步兵的机械化作战能力以配合坦克集团的高速突进能力，给步兵配备了汽车或运兵车。在实际作战中，由于步兵的防护能力较差，因此又开发出步兵可以搭乘作战的装甲车辆，以不间断配合坦克部队的行动，甚至安装重武器方便于巷弄间运行扫荡。1943年开始，为了鼓舞动摇的军心，希特勒用装甲掷弹兵命名那些搭乘装甲战斗车辆作战的步兵，希望用普鲁士军队的传统唤起军队的荣誉感。足见机械化步兵是伴随着坦克和大规模集团作战理论出现的兵种，其特点就是火力，机动力以及防护力强，适合于大规模的坦克集团作战。因此，第二次世界大战以后，在冷战时代，苏联和美国为首的两大军事集团重点发展了该兵种在核战争条件下的作战能力。

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

Mechanized infantry is distinguished from motorized infantry in that its vehicles provide a degree of protection from hostile fire, as opposed to "soft-skinned" wheeled vehicles (trucks or jeeps) for motorized infantry. Most APCs and IFVs are fully tracked, or are all-wheel drive vehicles (6×6 or 8×8), for mobility across rough ground. Some nations distinguish between mechanized and armoured infantry, designating troops carried by APCs as mechanized and those in IFVs as armoured.