His maiden post on the social media website was a video showing him engrossed in a Spider-Man comic titled The Return Of The Man Called Mysterio!
The actor officially joined Instagram on Wednesday and revealed he will be starring in the Spider-Man sequel.
In his first post, the 'Okja' actor also confirmed that he will take on the role of the villain Mysterio in the highly anticipated 2019 film 'Spider-Man: Far From Home'.
The Nightcrawler actor has always been rumoured to be portraying Quentin Beck / Mysterio - a key enemy of Spider-Man in the Marvel Comics - in the upcoming film, a sequel to 2017's Spider-Man: Homecoming.
The film will see Tom Holland return to play Spider-Man and Zendaya as Michelle Jones.
Jon Watts is returning as director, while Chris McKenna and John Francis Daley have penned the script. Kevin Feige and Amy Pascal are among the producers. The post is accompanied by the caption: "I just realised I'm not playing Spider-Man".
In the comic, Mysterio tries to convince Spidey he caused a man's death, The Hollywood Reporter reports, noting that it's unclear if this plot will carry over into the film.
Gyllenhaal was most recently seen onscreen in Jacques Audiard film The Sisters Brothers and in Paul Dano's Wildlife.
Spider-Man creators, Stan Lee and Steve Ditko, introduced Mysterio in the comics in 1964.
Before Far From Home, Spider-Man is expected to appear in the as-yet untitled Avengers 4, which will conclude the events of Infinity War.
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