The actor, best known for his role as Ron Weasley in the hit Harry Potter film franchise, joined the social media site on Wednesday (11 November) and shared a photo of his baby daughter, Wednesday.
Within four hours and one minute of joining the website, Grint had passed the million-follower mark.
Nature documentarian Attenborough joined the service earlier this year to help promote messages of environmentalism.
Attenborough took 284 minutes to get to the same total – 43 minutes longer than Grint.
Alongside the picture of Grint and his daughter, the actor commented: “Hey Instagram....only 10 years late, but here I am. Grint on the Gram! Here to introduce you all to Wednesday G. Grint. Stay safe, Rupert.”
Depp was asked to leave the film by Warner Bros following the verdict of his unsuccessful libel lawsuit against the publisher of The Sun newspaper.
An article in The Sun had described Depp as a “wife beater”. Judge Mr Justice Nicol ruled that the words were “substantially true”.
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