Marriagetoxin (Japanese: マリッジトキシン, Hepburn: Marijjitokishin; stylized as MARRIAGETOXIN) is a Japanese manga series written by Joumyaku and illustrated by Mizuki Yoda. It has been serialized on the mobile app and website of Shueisha's online magazine Shōnen Jump+ since April 2022, with its chapters collected in 15 tankōbon volumes as of October 2025. Viz Media licensed the series for English release. An anime television series adaptation produced by Bones Film is set to premiere in April 2026.

Hikaru Gero is a young man from the Poison Clan of professional assassins, which has been in existence for hundreds of years. Because he deals in the dark side of society and has never lived a "normal life", he considers himself to have no prospects with women and has no interest in marriage. However, one day, the Poison Clan states that if Gero does not marry and produce an heir to carry on their lineage, they will force his sister into bearing a child against her wishes. Wanting to save his younger sister from such a fate, Gero asks one of his assassination targets to marry him. When the cross-dressing marriage swindler Mei Kinosaki declines, Gero instead has Kinosaki train him to date and how to appeal to women with the goal of finding someone he truly wants to marry.

Marriagetoxin is written by Joumyaku and illustrated by Mizuki Yoda. The manga was officially announced by Shueisha on April 11, 2022, and began its weekly serialization for free on the Shōnen Jump+ mobile app and website on the 20th of the same month. Shueisha also simultaneously publishes the series in English for free on the online platform and mobile app Manga Plus.

In June 2023, Viz Media announced that they licensed the series for English release under the name Marriage Toxin.

These chapters have yet to be published in a tankōbon volume.

An anime television series adaptation was announced on October 1, 2025. It will be produced by Bones Film and directed by Motonobu Hori, with series composition and scripts handled by Kimiko Ueno, characters designed by Kōhei Tokuoka, who will also serve as chief animation director, and music composed by Taisei Iwasaki and Yuma Yamaguchi. The series is set to premiere in April 2026 on the brand new Ka-Anival!! programming block on Kansai TV, Fuji TV and their affiliates.

Marriagetoxin was nominated for the Next Manga Award in the Best Web Manga category in June 2022, and ranked eighth out of 50 nominees. It also ranked fifth in the Nationwide Publishers Recommended Comics of 2023. The series was recommended by Koyoharu Gotouge, Gege Akutami and Mogumo, the character designer for the anime series Mobile Suit Gundam: The Witch from Mercury.

The first volume received positive reviews for its art and action sequences. Hikaru Gero has been singled out as a "[v]ery relatable lead" with praise aimed at his design and his relationship with the "bubbly and energetic" Mei Kinosaki. Some criticism was aimed at Kinosaki's supporting role in the story rather than being a "fully realized character."