默克索引（Merck Index）是由美国默克公司出版的记录化学品、药物和生理性物质的综合性百科全书，收录超过一万多条有关个别的物质和其相关化合物的专题文章。本书亦于附录中收录有关有机化学的人名反应。默克索引亦设有订阅的电子检索形式，普遍被参考图书馆所采纳，以及在网上查阅的形式。

默克索引于1889年首次出版，至2013年已发行至15版。

默克索引的专题文章函盖范围包括：

The Merck Index is an encyclopedia of chemicals, drugs and biologicals with over 10,000 monographs on single substances or groups of related compounds. It also includes an appendix with monographs on organic named reactions. It was published by the United States pharmaceutical company Merck & Co. from 1889 until 2012, when the title was acquired by the Royal Society of Chemistry. An online version of The Merck Index, including historic records and new updates not in the print edition, is commonly available through research libraries.