方波是一种非正弦曲线的波形，通常会于电子和信号处理时出现。理想方波只有「高」和「低」这两个值。

在现实世界，方波只有有限的带宽。由于一般电子零件只有高（1）和低（0）两个值，方波就自然产生，并于数码开关电路中广泛应用。因为方波可以快速从一个值转至另一个（即0→1或1→0），所以方波就用作时钟信号来准确地触发同步电路。但是如果用频率定义域来表示方波，就会出现一连串的谐波。这可能会产生电磁波和电流脉波，影响周围的电路，产生杂讯和错误，对一些精密仪器如模拟数字数据转换器（analog-to-digital converter）影响十分明显，所以设计会使用正弦波作时钟信号来代替方波。

A square wave is a non-sinusoidal periodic waveform (which can be represented as an infinite summation of sinusoidal waves), in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. The transition between minimum to maximum is instantaneous for an ideal square wave; this is not realizable in physical systems. Square waves are often encountered in electronics and signal processing. Its stochastic counterpart is a two-state trajectory. A similar but not necessarily symmetrical wave, with arbitrary durations at minimum and maximum, is called a pulse wave (of which the square wave is a special case).