默克药厂（英文：Merck & Co., Inc.，NYSE：MRK），在美国与加拿大之外的地区称为默沙东药厂（Merck Sharp & Dohme，MSD），知名美国制药公司。该公司的前身是在德国达姆施塔特成立的伊曼纽尔·默克药厂（Emanuel Merck，也就是今日的默克股份有限公司（Merck KGaA），或又常称为「德国默克」）。1891年默克美国分公司成立，但如同许多在美国境内的德国企业资产一般，美国默克也于1917年第一次世界大战爆发后遭美国政府充公，并扶植其转型成为一独立公司继续经营。无论是按资本额抑或营业收入，默克都是世界上排名前七位的主要制药厂商。集团总部设于新泽西州联合县的凯尼尔沃思。

Merck & Co., Inc. (NYSE: MRK), d.b.a. Merck Sharp & Dohme (MSD) outside the United States and Canada, is an American pharmaceutical company and one of the largest pharmaceutical companies in the world.

The company was established in 1891 as the United States subsidiary of the German company Merck, which was founded in 1668 by the Merck family. Merck & Co. was nationalized by the US government during World War I and subsequently established as an independent American company in 1917. While it operates as Merck & Co. in North America, the original Merck based in Darmstadt holds the rights to the Merck name everywhere else. Merck & Co. is the world's seventh largest pharmaceutical company by market capitalization and revenue. Its headquarters is located in Kenilworth, New Jersey.