摩托化自行车或附发动机自行车是有动力的自行车，现在的摩托车即是从发动机自行车演变而来，摩托化自行车的发动机种类非常多，从二冲程引擎、四冲程引擎、电动机甚至蒸汽机的都有。电动自行车也是一种摩托化自行车。

最早的摩托化自行车装有发动机来帮助踩踏，那时汽油引擎与传动设计还在萌芽的阶段，其功率重量比很低，所以这种双重动力系统在此时特别有优势。随着技术的改进，踏板推进被二冲程或四冲程的汽油引擎取代，不过在一些汽机车与燃料都还是很贵的国家，摩托化自行车仍然很盛行。

现在的摩托化自行车有多种种类：

A motorized bicycle is a bicycle with an attached motor and transmission used either to power the vehicle unassisted, or to assist with pedaling. Since it always retains both pedals and a discrete connected drive for rider-powered propulsion, the motorized bicycle is in technical terms a true bicycle, albeit a power-assisted one. However, for purposes of governmental licensing and registration requirements, the type may be legally defined as a motor vehicle, motorcycle, moped, or a separate class of hybrid vehicle. Powered by a variety of engine types and designs, the motorized bicycle formed the prototype for what would later become the motorcycle.