The actor - who plays Tormund Giantsbane in the fantasy series - shared a lengthy statement on Instagram explaining he was self-isolating at home in Norway.
Sharing a selfie, he wrote: “Greetings from Norway! Sorry to say that I, today, have tested positive for COVID19, Coronavirus. My family and I are self-isolating at home for as long as it takes.
“We are in good health – I only have mild symptoms of a cold.”
The 41-year-old continued: “There are people at higher risk for who this virus might be a devastating diagnosis, so I urge all of you to be extremely careful; wash your hands, keep 1.5 meters distance from others, go in quarantine; just do everything you can to stop the virus from spreading.”
Kristofer’s Game Of Thrones co-star John Bradley, who plays Samwell Tarley, was one of the first to share their well wishes.
“Oh mate. Sending my love to you all. Get well,” he wrote.
The actor is the latest star to test positive for Covid-19.
On Monday Luther star Idris Elba revealed he had tested positive for the virus, whilst Good Morning Britain’s Susanna Reid announced she would be self-isolating after a member of her household developed symptoms.