受体拮抗剂（英语：receptor antagonist），也叫阻断剂（英语：blocker），是药理学术语，指能与受体结合，并能阻止激动剂产生效应的一类配体物质。拮抗剂对相应受体有亲和性，但没有性能，从而抑制了激动剂对受体的作用。拮抗剂可以结合受体的活性位点，也可以结合别构位点，甚至单独结合通常没有生物调节作用的位点来达到效果。拮抗剂和受体结合性质不同，所以结合时间有长有短，结合的可逆性也有不同。

拮抗剂分为竞争性拮抗剂和非竞争性拮抗剂等类型。大部分药物拮抗剂都与内源性配体或底物竞争受体结合位点来达到效果。

A receptor antagonist is a type of receptor ligand or drug that blocks or dampens agonist-mediated responses rather than provoking a biological response itself upon binding to a receptor. They are sometimes called blockers; examples include alpha blockers, beta blockers, and calcium channel blockers. In pharmacology, antagonists have affinity but no efficacy for their cognate receptors, and binding will disrupt the interaction and inhibit the function of an agonist or inverse agonist at receptors. Antagonists mediate their effects by binding to the active orthosteric (= right place) site or to allosteric (= other place) sites on receptors, or they may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity. Antagonist activity may be reversible or irreversible depending on the longevity of the antagonist–receptor complex, which, in turn, depends on the nature of antagonist–receptor binding. The majority of drug antagonists achieve their potency by competing with endogenous ligands or substrates at structurally defined binding sites on receptors.