谷氨酸脱羧酶（英语：Glutamate decarboxylase；GAD）是一个催化谷氨酸脱羧为γ-氨基丁酸并释放CO₂的酶。此酶使用磷酸吡哆醛作为一个辅因子。此反应以如下方式进行：

此酶在哺乳动物中存在着两种亚型，它们分别由两种不同的基因——Gad1与Gad2编码。这些亚型分别是GAD₆₇与GAD₆₅，分子量分别为67与65千道尔顿。GAD1与GAD2在大脑中被表达出来，在此处γ-氨基丁酸被用于作为神经递质，GAD2也会在胰腺中被表达出来。

至少有两种更多的类型，GAD25与GAD44（胎儿；EGAD）被发现于发育中的大脑中。

Glutamate decarboxylase or glutamic acid decarboxylase (GAD) is an enzyme that catalyzes the decarboxylation of glutamate to GABA and CO₂. GAD uses PLP as a cofactor. The reaction proceeds as follows:

In mammals, GAD exists in two isoforms encoded by two different genes - GAD1 and GAD2. These isoforms are GAD₆₇ and GAD₆₅ with molecular weights of 67 and 65 kDa, respectively. GAD1 and GAD2 are expressed in the brain where GABA is used as a neurotransmitter, GAD2 is also expressed in the pancreas.