保护基是有机合成上的一个概念。是使有机分子中特定官能团发生希望的反应，同时抑制其他官能团的反应而通过特定试剂将一部分官能团可逆地转化成惰性基团的策略。有机合成中常需要保护的官能团有羟基、氨基、羰基等等。保护基的使用提高了有机合成的可操作性和准确性，使人们能够合成更加复杂的分子，但是上保护基、去保护基同时也降低了反应的总产率。避免使用保护基一直是合成家的梦想，但是“在可预见的未来，如同死亡和税收一样，保护基的使用是有机合成中不可避免的东西。”

A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.

In many preparations of delicate organic compounds, some specific parts of their molecules cannot survive the required reagents or chemical environments. Then, these parts, or groups, must be protected. For example, lithium aluminium hydride is a highly reactive but useful reagent capable of reducing esters to alcohols. It will always react with carbonyl groups, and this cannot be discouraged by any means. When a reduction of an ester is required in the presence of a carbonyl, the attack of the hydride on the carbonyl has to be prevented. For example, the carbonyl is converted into an acetal, which does not react with hydrides. The acetal is then called a protecting group for the carbonyl. After the step involving the hydride is complete, the acetal is removed (by reacting it with an aqueous acid), giving back the original carbonyl. This step is called deprotection.