Mary Jane "MJ" Watson is a character appearing in American comic books published by Marvel Comics. The character was created by Stan Lee and Steve Ditko, making her first appearance in The Amazing Spider-Man #25 (June 1965), and subsequently designed by John Romita Sr. in #42 (November 1966). Since then, she has gone on to become Spider-Man's main love interest and later his wife (as Mary Jane "MJ" Watson-Parker) before their marriage was sold to Mephisto; Mary Jane has also served as a supporting character to Iron Man and Venom.

Although she made a brief first appearance in The Amazing Spider-Man #25 with a plant obscuring her face, as part of a then-long-running recurring gag about Aunt May attempting to set Peter up with her friend's "nice girl" niece, Mary Jane's first official face reveal was a cameo appearance in The Amazing Spider-Man #42. Designed and drawn by John Romita Sr., her entrance is regarded as one of the most iconic introductions in comic history, owing to its build-up, her hyper-vibrant red hair and beauty, and her introductory line, "Face it, Tiger… you just hit the jackpot!". Since then, 'Tiger' has been her most recognizable nickname for Peter, spanning comics and media adaptations.

Throughout her initial appearances, Mary Jane was written as a foil to Peter's initially intended soulmate, Gwen Stacy, with her extroverted, fun-loving personality (a mask for her troubled home life) contrasting with Gwen being more like Peter in demeanor and intellect. Nonetheless, following Gwen's death in "The Night Gwen Stacy Died" story arc, the heartbroken Mary Jane became more caring and empathetic, and eventually one of the few people to consistently know Peter's secret identity. This would result in the two falling deeply in love and eventually getting married, before their marriage was undone in the storyline "One More Day" due to the timeline manipulations by Mephisto. Unbeknownst to them, they are further destined to have a daughter who will end Mephisto's eventual reign over the Earth, whom Mephisto seeks to erase from reality. In Spider-Man/Red Sonja, she becomes Red Sonja, in Armed and Dangerous, she becomes the third Jackpot, while in All-New Venom, she becomes the new host of Venom. In the alternate universes of the Marvel Multiverse, Mary Jane is depicted as the superheroines Spider-Woman in Exiles (as a lesbian) and Marvel Mangaverse (as straight), and Spinneret in Amazing Spider-Man: Renew Your Vows, as well as Carnage in Spider-Gwen.

Since her debut, Mary Jane Watson has been described as one of Marvel's most notable non-powered female characters. In film, Kirsten Dunst portrayed the character in Sam Raimi's Spider-Man trilogy, while Shailene Woodley had an uncredited silent cameo appearance as the character in the 2014 film The Amazing Spider-Man 2; Zoë Kravitz voiced Mary Jane in the 2018 animated film Spider-Man: Into the Spider-Verse, with Nicole Delaney and Melissa Sturm voicing other versions of the character in its 2023 sequel Spider-Man: Across the Spider-Verse.

Mary Jane Watson is mentioned in The Amazing Spider-Man #15 (August 1964), and is initially used as a running joke of the series, as Peter Parker's Aunt May repeatedly attempts to set her unwilling nephew up on a date with her. Parker (also known as Spider-Man) consistently worms his way out of meeting Mary Jane who, aside from a brief appearance in #25 (June 1965) with her face obscured, is never actually seen until The Amazing Spider-Man #42 (November 1966). Peter David wrote in 2010 that artist John Romita Sr. "made the definitive statement of his arrival by pulling Mary Jane out from behind the oversized potted plant [that blocked the readers' view of her face in issue #25] and placing her on panel in what would instantly become an iconic moment". Romita has stated that in designing Mary Jane, he "used Ann-Margret from the movie Bye Bye Birdie as a guide, using her coloring, the shape of her face, her red hair and her form-fitting short skirts".

According to co-creator Stan Lee, he and Romita had intended for Gwen Stacy to be Spider-Man's one true love, and introduced Mary Jane "just for fun", but that "somehow, Mary Jane seemed to have all the personality, and much as we tried to make Gwen more attractive, we couldn't! We, ourselves, felt that Mary Jane ended up being not only more beautiful but more fun and more interesting, and we finally decided to let Peter end up with her, but it was… as though the characters had taken over!".

Mary Jane Watson's unexpected popularity with readers after her debut changed the course of the plan for Gwen Stacy's character, who was intended to be the true love for Peter and was characterized as a sweet Daddy's girl in contrast to Mary Jane's '60s counterculture, vampy and liberated personality. Fans surprisingly liked Mary Jane more and demanded for her to replace Gwen's role as Peter Parker's main love interest as readers found Mary Jane more exciting and that "no matter how we [i.e. Lee and his artist/co-plotter collaborators] wrote it, Mary Jane always seemed more interesting!". Mary Jane's flamboyance and 'life of the party' personality (a mask in reality) were written as a contrast to Gwen, being more on Peter's intellectual and emotional wavelength. However, following Gwen's death in "The Night Gwen Stacy Died" story arc, the character was reworked to be more emotionally grounded and open-hearted.

The names "Mary Jane" and "MJ" are also common slang terms for marijuana. When asked about this, Stan Lee claimed it was purely coincidental, that he knew nothing about drugs and had never tried marijuana.