The 33-year-old actor is best known for his starring roles in box office hits such as 21 Jump Street and Knocked Up.
Fans may also remember him sporting a much fuller figure in 2007 hits like Superbad.
Pictured out this weekend, Jonah is almost unrecognisable having shed a serious amount of weight since gaining the pounds for his 2016 blockbuster movie War Dogs.
He was photographed leaving a Los Angeles gym on Friday afternoon wearing a navy blue tank top, black shorts, high socks and trainers and clutching a pink healthy shake.
But how did the Hollywood star manage such a dramatic slim down?
Jonah’s weight loss journey began in 2011 when he was filming Moneyball with Brad Pitt.
Rumours circulated Brad was the driving force behind his fitness overhaul, but Jonah later came out and said his weight loss was simply triggered by a new diet and looking after himself.
He told ABS News: “It was mostly diet. I wish there was some crazy thing that I did, like a pill or a genie or something, but I went to see a nutritionist, and he told me what to eat and to change my habits and stuff.
“I found that Japanese food was very helpful to me.”
The actor also revealed the key to his weight loss was giving up beer.
He told The Kyle and Jackie O Show: “I went to a nutritionist and I said, ‘I’ll lose weight, I’ll eat healthier and whatever, but figure out how I can drink beer’.
“It’s so annoying because when I don’t drink beer, I get really really thin, Then when I drink beer, I get a little bigger.”
Two years ago, Jonah put weight back on for his role in the movie War Dogs, which he starred in with Channing Tatum.
When he appeared on The Tonight Show with Jimmy Fallon he said: “I gained weight for this movie War Dogs, and then I wanted to get in better shape, so I called Channing Tatum, and said, ‘Hey, if I eat less and go to a trainer, will I get in better shape?
“And he said, ‘Yes, you dumb motherf*****, of course you will, it’s the simplest thing in the entire…