溢洪道（英语：spillway），系指水坝或堰的构造物，功能在于排除宣泄超过水库容量之多余洪水，防止溢顶而破坏坝体，以确保水坝的安全。依照溢流泄方式之不同，溢洪道可区分为自由溢流式溢洪道（overflow spillway）及闸门控制式溢洪道；又依照泄水构造物形式，可分为隧道式溢洪道、陡槽式溢洪道（chute spillway）、钟口型溢洪道（bell-mouth spillway 或 morning glory spillway，又称为喇叭形溢洪道）、侧槽溢洪道（side channel spillway）、管流式溢洪道（pipe spillway）、虹吸式溢洪道（siphon spillway）、坝堰式溢洪道等。。溢洪道并不一定建造在坝身建筑物上，亦可在坝址附近另行兴建溢洪道，例如侧槽溢洪道即离开坝身。

A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy the dam. Except during flood periods, water does not normally flow over a spillway. In contrast, an intake is a structure used to release water on a regular basis for water supply, hydroelectricity generation, etc. Floodgates and fuse plugs may be designed into spillways to regulate water flow and dam height. Other uses of the term "spillway" include bypasses of dams or outlets of a channels used during highwater, and outlet channels carved through natural dams such as moraines.