Merriam-Webster, Incorporated is an American company that publishes reference books and is mostly known for its dictionaries. It is the oldest dictionary publisher in the United States.

In 1831, George and Charles Merriam founded the company as G & C Merriam Co. in Springfield, Massachusetts. In 1843, after Noah Webster died, the company bought the rights to An American Dictionary of the English Language from Webster's estate. All Merriam-Webster dictionaries trace their lineage to this source.

In 1964, Encyclopædia Britannica, Inc., acquired Merriam-Webster, Inc., as a subsidiary. The company adopted its current name, Merriam-Webster, Incorporated, in 1982.

In 1806, Webster published his first dictionary, A Compendious Dictionary of the English Language. The following year, in 1807, Webster began two consecutive decades of intensive work to expand his publication into a fully comprehensive dictionary, An American Dictionary of the English Language. To help trace the etymology of words, Webster learned 26 languages. Webster hoped to standardize American speech, since Americans in different parts of the United States used somewhat different vocabularies and spelled, pronounced, and used words differently.

In 1825, while spending a year abroad at the University of Cambridge and then in Paris, Webster completed this dictionary, which featured 70,000 words, about 12,000 of which had never before appeared in a dictionary. A spelling reformer, Webster believed that English spelling rules were unnecessarily complex and used the dictionary to introduce American English spellings, replacing colour with color, waggon with wagon, and centre with center. He also added American words, including skunk and squash, that did not appear in British dictionaries. Three years later, at age of 70 in 1828, Webster published the dictionary. But the dictionary proved a commercial disappointment, selling only 2,500 copies and leaving him in debt. In 1840, however, he released a second edition, which was published in two volumes and proved a vastly greater commercial success.

In 1843, following Webster's death, George Merriam and Charles Merriam secured publishing and revision rights to Webster's 1840 dictionary. Four years later, in 1847, they published a revised version of it, which did not change any of the main text but added new sections. In 1859, they published a second update with illustrations. In 1864, Merriam published a greatly expanded edition, which was the first version to change Webster's text, largely overhauling his work yet retaining many of his definitions and the title, An American Dictionary. In 1884, the edition contained 118,000 words, "3000 more than any other English dictionary".

In 1890, they published a dictionary, which they retitled Webster's International. The vocabulary was vastly expanded in Webster's New International editions published in 1909 and 1934, featuring over half a million words. Their 1934 edition was retrospectively called Webster's Second International, or simply "The Second Edition" of the New International.

In 1898, Collegiate Dictionary, now in its eleventh edition, was introduced. In 1890, following publication of Webster's International. two Collegiate editions were issued as abridgments of each of their Unabridged editions.