电阻器（Resistor），泛指所有用以产生电阻的电子或电机配件。电阻器的运作跟随欧姆定律，其电阻值定义为其电压与电流相除所得的商数。

其中

电阻器是电子电路中常见的组件，实际的电阻可以由许多不同的材质构成，包括薄膜、水泥或是高电阻系数的镍铬合金（电阻丝）。电阻器也可集成到集成电路中，特别是模拟IC，也可以集成到混合式集体电路或印刷电路中。

电阻器的机能可以用其电阻来表示，常用的电阻器阻值范围超过9个数量级。电阻器阻值有一定的误差范围，在电子电路中使用电阻器时，需考虑使用电阻器的允许误差和应用是否符合，若是一些精密的电路，可能也需要考虑电阻器的温度系数。电阻器也会标示其最大功率，此数值需大于电阻器在电路中预期的能量消耗，尤其在电力电子应用中更需考虑。大功率的电阻器一般会需要散热片。在高压电路中也需考虑电阻器可承受的最大电压，电阻器的工作电压一般没有下限，但电阻器的电压若超过其最大电压，可能在电流流过时使电阻器燃烧。

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. Resistors may be used to reduce current flow, and, at the same time, may act to lower voltage levels within circuits. In electronic circuits, resistors are used to limit current flow, to adjust signal levels, bias active elements, and terminate transmission lines among other uses. High-power resistors, that can dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity.