A motorized bicycle is a bicycle with an attached motor used to assist with pedalling. Generally considered to be a vehicle, sometimes as a motor vehicle or a class of hybrid vehicle, motorized bicycles are usually powered by electric motors or small internal combustion engines. Some can be propelled by the motor alone if the rider chooses not to pedal, while in others the motor will only run if the rider pedals.

Some early motorized bicycles were powered by internal combustion engines whereas some utilized electric motors. With lighter batteries and better storage density, the electric motor has recently seen an increase in popularity.

Motorized bicycles are distinguished from motorcycles by being capable of being powered by pedals alone if required. The actual usage of the pedals varies widely according to the type of vehicle. Those known as mopeds mostly have pedals for emergency use or because of legal requirements and these are not normally used. Those known as power-assist bikes have the pedals as the main form of propulsion with the motor used to give a bit of extra speed, especially uphill. Many motorized bicycles are based on standard bicycle frame designs and technologies, although the modifications to the design to support motorization may be extensive.

In countries where there is a strong bicycle culture (notably in Asia), the motorized bicycle is particularly popular; in 1996 Shanghai had 370,000 motorized bicycles and 470,000 other vehicles.