The serial comma (also referred to as the series comma, Oxford comma, or Harvard comma) is a comma placed after the second-to-last term in a list (just before the conjunction) when writing out three or more terms. For example, a list of three countries might be punctuated with the serial comma as "France, Italy, and Spain" or without it as "France, Italy and Spain". The serial comma can help avoid ambiguity in some situations, but can also create it in others. There is no universally accepted standard for its use.

The serial comma is popular in formal writing (such as in academic, literary, and legal contexts) but is usually omitted in journalism as a way to save space. Its popularity in informal and semi-formal writing depends on the variety of English; it is usually excluded in British English, while in American English it is common and often considered mandatory outside journalism. Academic and legal style guides such as the APA style, The Chicago Manual of Style, Garner's Modern American Usage, Strunk and White's The Elements of Style, and the U.S. Government Printing Office Style Manual either recommend or require the serial comma, as does The Oxford Style Manual (hence the alternative name "Oxford comma"). Newspaper stylebooks such as the Associated Press Stylebook, The New York Times Style Book, and The Canadian Press stylebook typically recommend against it. Most British style guides do not require it, with The Economist Style Guide noting most British writers use it only to avoid ambiguity.

While many sources provide default recommendations on whether to use the serial comma as a matter of course, most also include exceptions for situations where it is necessary to avoid ambiguity (see Serial comma § Recommendations by style guides).

The comma itself is widely attributed to Aldus Manutius, a 15th-century Italian printer who used a mark—now recognized as a comma—to separate words. Etymologically, the word comma, which became widely used to describe Manutius's mark, comes from the Greek κόμμα (lit. 'to cut off'). The serial comma has been used for centuries in a variety of languages, though not necessarily in a uniform or regulated manner.

The serial comma is most often attributed to Horace Hart, the printer and controller of the Oxford University Press from 1893 to 1915. Hart wrote the eponymous Hart's Rules for Compositors and Readers in 1905 as a style guide for the employees working at the press. The guide called for the use of the serial comma, but the punctuation mark had no distinct name until 1978, when Peter Sutcliffe referred to the serial comma as such in his historical account of the Oxford University Press.

Sutcliffe, however, attributed the serial comma not to Horace Hart but to F. Howard Collins, who mentioned it in his 1905 book, Author & Printer: A Guide for Authors, Editors, Printers, Correctors of the Press, Compositors, and Typists.

Common arguments for the consistent use of the serial comma are:

Common arguments against the consistent use of the serial comma are: