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The Harry Potter star shared his first post last week, a photo of his baby daughter he recently welcomed to the world with his partner Georgia Groome. The actress is best known for her role in the teen movie hit Angus, Thongs, and Perfect Snogging.
"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," he posted.
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Previous record holders include Jennifer Aniston, Pope Francis, and the Duke and Duchess of Sussex.
Grint currently has over 3.3 million followers and has a bit of catching up to do if he wants to meet Attenborough's follower count of 6 million.
Among the 80 accounts Grint follows on the platform include New Zealand director Taika Waititi.
The Harry Potter cast recently reunited online. During the Instagram event hosted by Tom Felton, who played Draco Malfoy in the films, Grint shared some of his favourite memories from set.
He said he ruined a number of takes by accidentally giggling.
"I remember on Potter it would always be the most inappropriate scenes, like Dumbledore's funeral was a particularly bad one.
"For some reason, I found that absolutely hilarious. And yeah, once you start laughing, it's very hard to stop."
When the star turned 30 in 2018, he told the Guardian that he wanted to have children.
"Turning 30 felt strange. It just doesn't feel like I'm there yet and I don't know what the future holds.
"I'm just going to go with the flow, keep playing interesting characters and see what happens.
"I'd like to settle down and have kids soon. If I had a son, would I call him Ron?"