Sinfest is a long-running American webcomic by Tatsuya Ishida. Updating daily, Sinfest started as a black comedy strip in January 2000. It has featured a wide range of perspectives over its long history, including on American politics, organized religion, and radical feminism. Shaenon Garrity's 2012 review in The Comics Journal observed that "raunchy strips about strippers are followed by cute cat-and-dog gags are followed by religious humor are followed by autobio strips are followed by shit-stirring political cartoons are followed by spoken-word poetry are followed by lessons in drawing Japanese kanji, one of Sinfest’s signature running features", and that "Sinfest is always, first and foremost, about what Ishida wants to cartoon at any given moment."

Sinfest originated as a four-panel comedy strip relying on dark humor with frequent pop culture references. Over its first decade it evolved into a more serious work, with a large cast of regular characters commenting on such themes as organized religion, American exceptionalism, and economic insecurity. It abruptly shifted focus to radical feminism in 2011, tackling issues such as slut-shaming, misogyny, and street harassment.

In an interview with Publishers Weekly, Ishida stated that he knew he wanted to become a comics author ever since he read a Peanuts paperback as a child: "[S]omething about the simplicity and solitary nature of the medium appealed to me." Ishida briefly served as penciller for Dark Horse Comics' G.I. Joe Extreme in the early 1990s. Ishida said that he botched this job, noting that "several [of his] pages were so poorly drawn they had to get another guy to redo them entirely".

In 2000, Ishida taught himself HTML, put together a Geocities web page, and started uploading Sinfest strips seven days per week. Ishida stated that he managed to sustain this strict schedule during the first seven years purely through "coffee and revenge". Ishida is rather private and has little interaction with his readership.

Over the years, Sinfest has gone through many shifts in tone. Ishida views his older works as an indicator of his emotional state during that period, describing his early Sinfest strips as "unhinged, totally off the chain". In 2009, Ishida claimed his strip was "still pretty wild, but there's also more warmth, more tenderness", citing 2005 as a turning point towards more sentimental, character-driven storylines. Sinfest was nominated for three Web Cartoonists' Choice Awards in 2004.

Ishida self-published three print volumes of Sinfest between 2002 and 2005. Two volumes of early Sinfest have been published in print by Dark Horse Comics. The first of these was released in mid-2009 and reprints the entire first year of the webcomic. The second volume, a 2011 collection titled Viva la Resistance, covers the webcomic's run from 2003 to 2004, featuring over 600 pages that were previously uncollected. Sinfest has also appeared in the Norwegian comic magazine Nemi.

During the 2008 United States presidential election, Sinfest incorporated more political themes. Ishida stated that he switches between characters and situations in his webcomic "pretty much on a whim", claiming that the longer storylines of his webcomic help to tie it all together. In 2011, Ishida started to produce weekly colored strips, giving readers "something extra fun and engaging" on Sundays.

In October 2011, the comic abruptly shifted in tone, focusing heavily on radical feminist themes. Ishida introduced new characters to explore these new themes, and to confront the humor in older strips. PC Magazine listed Sinfest among the best webcomics of 2015.

By 2024, the strip had changed direction again; author Sean Kleefeld said that when catching up on Sinfest issues, he "wasn't understanding them", and that the comic's political themes had gone on a "downward spiral".

• Sinfest website