Monthly Afternoon (Japanese: 月刊アフタヌーン, Hepburn: Gekkan Afutanūn) is a Japanese monthly seinen manga anthology published by Kodansha. The first issue was published in December 1986, with new issues released on the 25th of each month. Monthly Afternoon has spawned many successful manga series that span a variety of different genres, such as Oh My Goddess!, Blade of the Immortal, Genshiken, and Big Windup!. Chapters of each manga series in the magazine are compiled and released as tankōbon volumes under its "Afternoon KC" imprint. A spin-off magazine, named good! Afternoon, started publishing in November 2008.

The magazine was founded as a sister magazine to Morning by the same publisher, Kodansha. It originated as a special issue to the magazine, titled Morning Magnum Zōkan. Its first issue was released on December 25, 1986, with the cover date of February 1987. Its purpose was as a complementary magazine, acting as an "experimental zone" for new artists to develop unique art styles in manga. Vice editor of Morning, Yasumitsu Tsutsumi, described the differences between the two magazines in 1994: "Morning uses manga to express ideas and write stories. Afternoon, of course, is similar, but it is also about manga itself, and experimenting and developing forms of expression in manga."

Initially, both Afternoon and Morning shared the same editorial department. Its first editor-in-chief was also the editor-in-chief of Morning at the time, Yoshiyuki Kurihara. According to Frederik L. Schodt, stories that the editors of Morning liked but felt did not fit the magazine would often land in Afternoon. Afternoon has since maintained its own separate editorial department.

Under the magazine's approach of eschewing an editorial policy in favor of publishing what is experimental and "interesting", Afternoon eventually gained a reputation for its sheer quantity of both its titles and pages, alongside its diverse selection of manga. From 1992 to 1997, an average issue would contain over 1,000 pages. Adding to its diversity was the novel approach of inviting foreign artists and writers to create manga in Afternoon, much like in Morning.

Many of the artists working for the magazine used to publish amateur doujinshi and were influenced by lolicon amateur manga. Sharon Kinsella claims that around half of all series featured in Afternoon between 1994 and 1997 were inspired by lolicon aesthetics. She lists Discommunication, Gunsmith Cats, Assembler 0X, Seraphic Feather, Aqua and Oh My Goddess! as examples.

A sister magazine, titled Afternoon Season Zōkan [ja], was originally published in eight issues between 1987 and 1988, shortly after Afternoon's launch. After over a decade, it was revived and began publishing on a quarterly basis, from October 1999 to October 2002. After 14 issues, the magazine was discontinued; some of its series, like Mushishi and Mokke, were transferred to Afternoon. Another sister magazine, good! Afternoon, began publishing on November 7, 2008.

Between 2006 and 2009, Afternoon was part of Kodansha's "e-1day" series, a comprehensive website which also included Morning and Evening, its other sister magazine. A similar website, "Moae", was established in May 2013, featuring the aforementioned magazines and their spin-offs, Morning Two and good! Afternoon. It has since been discontinued. An online platform, titled Comic Days [ja], was launched in March 2018, featuring manga titles from various Kodansha magazines, including Afternoon. As of November 2013, new issues of Afternoon are distributed simultaneously in print and digitally.

In February 2024, the Afternoon editorial department, Skip and Loafer author Misaki Takamatsu, and the Skip and Loafer anime team donated 10 million yen to the Ishikawa Prefecture Relief Fund after the 2024 Noto Earthquake.