Seinen manga is a category of Japanese comics marketed towards young adult men between the ages of 18 and 40. The term "seinen" literally means "youth" in Japanese, but it is also used to describe the target audience of magazines like Weekly Manga Times and Weekly Manga Goraku, which cater specifically to mens interests. Seinen manga are distinguished from shōnen manga, which are for young teen boys, although some seinen manga share similarities with shōnen manga.

Seinen manga can focus on a wide range of genres, including action, politics, science fiction, fantasy, relationships, sports, or comedy. There is a stronger focus on plot and character development, and consequently less focus on action, than in shōnen manga. Characters and their interactions are often more developed than in shōnen anime, and the themes are more psychological, satirical, violent, sexual, and realistic.

A common way to tell if a manga is seinen is by looking at whether furigana is used over the original kanji text: if there are furigana on all kanji, the title is generally aimed at a younger audience. The title of the magazine it was published in is also an important indicator. Usually, Japanese manga magazines with the word "young" in the title (Weekly Young Jump, for instance) are seinen.

In summary, seinen manga is a demographic category of Japanese comics marketed towards young adult men between the ages of 18 and 40. It is distinguished from shōnen manga by its target audience and its focus on plot and character development. Seinen manga can cover a wide range of genres and are often more mature and realistic than shōnen manga.