According to The Sun, the Fifty Shades of Grey star has replaced model Kate Moss as the face of the popular low-calorie cola drink and has already filmed his ad campaign at an outdoor pool in London.
It means the Holywood actor will soon be the face on TV screens and advertisements for the drink.
According to the newspaper, the deal is worth “millions” for the 42-year-old actor who also starred in BBC thriller The Fall.
A source reported by the newspaper said: “Jamie shot a huge campaign for Diet Coke just recently at a lido in south London.
“This is going to be absolutely massive and Coca Cola has thrown a lot of money at it.
“Jamie’s deal is worth millions and they think it’s going to pay dividends with giving fans a bit of nostalgia.
“As well as a TV advert with Jamie, there will be a massive push on billboards and social media.”
Last month, Dornan – who is married to Amelia Warner with three children – revealed he is “the happiest I have ever been”.
During a conversation on golfing podcast On The Road With Iona, Dornan said: “I’m so happy with my family; I married an incredible woman and we have three healthy girls and I love that stability and I’m thankful for it,” he said.
“Career-wise I’m also the happiest I’ve ever been, getting to do really cool work and work with cool people.”
Dornan also paid tribute to Northern Ireland and growing up in the country for staying grounded in the film and TV industry.
“I’m so grateful for where I’m from, it’s a big part of me, and I really care about that part of the world,” he said.
“I think it’s been beneficial to me to be from Northern Ireland and to have an environment where ego isn’t allowed. I’m not saying it doesn’t exist, but it’s not received well, people don’t like it.”
“We’re a real culture of putting each other down in a fun and grounding way and it’s super important I think,” he added.
“I’ve had the same group of friends since I was a kid and it’s a total necessity for keeping your head, especially in the line of work I’m in.”
He told host Iona that, while he doesn’t regret starring in the 50 Shades movies, he said the criticism helped him “tune out the noise” that comes with big roles in Hollywood.
“I don’t regret the movies at all but there was a lot of awful stuff said about the movies and about me at that time,” he added.
“It’s just noise though and you have to find a way of turning that noise down and try and live a normal life.
