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 name reactions. It is published by the United States pharmaceutical company Merck & Co. The Merck Index is also available by subscription in an electronic searchable form, commonly carried by research libraries, as well as in a web-accessible form.

The current edition is the 14th, released in October 2006.

Monographs in the Merck Index typically contain:

• a CAS registry number
• synonyms of the substance, such as trivial names & International Union of Pure and Applied Chemistry nomenclature
• a chemical formula
• molecular weight
• Percent composition
• a structural formula
• a description of the substance's appearance
• melting point and boiling point
• solubility in solvents commonly used in the laboratory
• citations to other literature regarding the compound's chemical synthesis
• a therapeutic category, if applicable