除草剂又称杀草剂，是一类用来杀死植物的药剂，全球约有233种。这些药剂能够选择性地作用于特定目标，使其他对于人类有用的农作物不受伤害，或受的伤害较小。有些除草剂能妨碍杂草的生长，这类除草剂通常以植物激素为基础。用来清理荒废土地的除草剂能够杀死所有与其接触的植物。有些植物本身也有制造除草剂的能力，例如胡桃。

除草剂被广泛使用于农业以及草坪管理，或是用来控制公路与铁路上的植被生长。也有一些用来管理森林、牧草，以及管控野生生物的活动区域。

Herbicide(s), also commonly known as weedkillers, are chemical substances used to control unwanted plants. Selective herbicides control specific weed species, while leaving the desired crop relatively unharmed, while non-selective herbicides (sometimes called "total weedkillers" in commercial products) can be used to clear waste ground, industrial and construction sites, railways and railway embankments as they kill all plant material with which they come into contact. Apart from selective/non-selective, other important distinctions include persistence (also known as residual action: how long the product stays in place and remains active), means of uptake (whether it is absorbed by above-ground foliage only, through the roots, or by other means), and mechanism of action (how it works). Historically, products such as common salt and other metal salts were used as herbicides, however these have gradually fallen out of favor and in some countries a number of these are banned due to their persistence in soil, and toxicity and groundwater contamination concerns. Herbicides have also been used in warfare and conflict.