Series fiction refers to a group of independently published works of fiction that are related to one another, usually through similar elements of setting and characters. A common example of series fiction is a book series. Series fiction spans a wide range of genres, and is particularly common in adventure, mystery, romance, fantasy, and science fiction. While commonly associated with children's and young adult literature, series fiction has also been a significant feature of mainstream and genre fiction for adults. Early examples include James Fenimore Cooper's Leatherstocking Tales (1823) and Honoré de Balzac's La Comédie humaine (1829).

Typically authored by a single writer, some series are collaborative efforts driven by publishers, as seen in The Hardy Boys or Sweet Valley High. Series fiction can be classified into progressive series, where characters grow and narratives are chronological, or successive series, which consist of self-contained stories that can be read in any order. The popularity of series fiction has grown with the commercialization of publishing, the rise of fan communities, and the expansion into multimedia franchises. Series fiction has been criticized for its formulaic plots and perceived lack of literary value, particularly in series aimed at younger and female audiences.

Stories that share certain elements, like characters, have a long tradition. For example, William Shakespeare reused the character of John Falstaff in several of his plays. Victor Watson attributes the origin of series fiction to the United States. One of the earliest series fiction, and according to Watson the first American one, was the Leatherstocking Tales series by James Fenimore Cooper, which began in 1823 with The Pioneers. Some of these first appeared in the form of dime novels, and many were aimed at less demanding readers, including youths. Outside the United States, Watson names Margaret Sidney's Five Little Peppers series (debuting in 1881 with Five Little Peppers and How They Grew), which he considers the first series fiction in Australia and E. Nesbit's Bastable series (debuting in 1899 with The Story of the Treasure Seekers), which he called the first in Britain. In India, he pointed to Satyajit Ray's Feluda detective series that began in 1961.

However, Peggy Lin Duthie identified much earlier examples of series fiction in Britain, predating the American series claimed by Watson to be the earliest. She considers the earliest example to be Sir Walter Scott's Waverly Novels, which begun in 1814 with Waverley. Before E. Nesbit, she also identifies other British authors of series fiction, such as Anthony Trollope with his Chronicles of Barsetshire (which started in 1855 with The Warden), and Margaret Oliphant's The Chronicles of Carlingford (begun in 1861 with "The Executor", a short story, and a novel, The Rector)

As for non-English language examples, Watson mentioned German writer Erich Kästner with his Emil and the Detectives series (1929), and French writer Paul Berna with his A Hundred Million Francs (1955). Duthie again identified an earlier French example, the mainstream series of Honoré de Balzac, La Comédie humaine, that debuted in 1829.

In the interwar period, series fiction expanded from literature (novels) to other mediums, notably comics and films. During that time, Edward Stratemeyer's Tom Swift series was extremely popular among young readers in the United States. The postwar period saw the emergence of a female protagonist in series fiction. The use of female protagonists during this period is often criticized for being one-dimensional, with a focus solely on beauty. The popularity of series fiction increased during the internet era, from the late 1990s onward.

Victor Watson defined series fiction broadly as "a sequence of related stories about the same groups of characters, usually by the same author", as well as "a series of narratives, published separately, often over a considerable period of time, mostly about the same characters, and usually written by one author". He notes that its key characteristics that distinguish it from other types of fiction is its "extended overall length and its composition in separate narratives". Watson although noted that there are notable exceptions to his definition. While the unifying feature for most series are characters, for some, that can be a concept, a place or an institution. For example, the Greyfriars School series by Charles Hamilton, the Chalet School series by Elinor Brent-Dyer or the Green Knowe series by Lucy Boston feature changing characters, and are connected through a particular place; while Lucy Fitch Perkins's Twins series had a theme of featuring twins, with characters, countries and historical periods varying between the installments. Likewise, while many series are primarily written by a single author, there are some notable series written by various writers (such as the Twins series, The Hardy Boys series, the Bobbsey Twins series, the Sweet Valley High or the Animal Ark series). Watson classifies them as "publisher's format series".

While series are common in genre fiction, they have also been found in mainstream fiction (as early as with Honoré de Balzac's La Comédie humaine from 1829). Series fiction is often incorrectly pigeonholed into one primary genre, but in fact many series can have elements of different genres; those genres can also change over time as authors mature or experiment with different styles. A long running series that was set in times contemporary to the readers can be seen as historical if the time in the series progresses very slowly, and it becomes long-running (ex. Alison Uttley's Little Grey Rabbit series begun in the 1920s and continued into the 1970s, with later books receiving explanatory notes about history).