Taking to Instagram on Monday, Sam revealed that it's been a decade since he was cast as the leading role in the fantasy series.
The actor celebrated the news by sharing a photo from the drama showing Sam dressed in character as Jamie. He wrote in the caption: "Happy castaverssary JAMMF! 10 years ago was lucky to be given the opportunity to be custodian of the ginger wig…@outlander_starz."
Fans flooded to the comments section to wish the 43-year-old a happy anniversary, with one person writing: "Thanks Sam for taking Jamie Fraser from the pages of a book and giving life to him for us all to enjoy. Happy 10th Castaversary!!" while another added: "The best casting decision ever made. Thank you for bringing this beloved character to life in such a special way. Congratulations on all of your hard work over these years."
Amongst the fans congratulating the star for his ten years in the role was his co-star and real-life friend, Caitríona Balfe.
Taking to the comments section, the Irish actress penned a sweet tribute to her on-screen husband that read: "The man, the myth, the legend …!!!! Happy Castaversary darling!!! (I may have been slightly premature a few days ago!!) So happy I got to go along on this insanely amazing journey with you. Much love, the old ball and chain x."
Fans were quick to express their delight over Caitríona's message, with one person writing: "I've never seen a friendship like it before. It's a lovely thing," while another commented: "@caitrionabalfe so beautiful. Best TV couple, we are so lucky with you both."
While Sam and Caitríona are certainly a winning duo on-screen, they also enjoy a close off-screen friendship and have often spoken about it in interviews.
In a previous interview with Elle, the Belfast actress said: "Sam's just so kind. He's a really good friend. He always checks in. I don't know, he has one of the biggest hearts and smallest egos that I know.
"I feel very lucky. Sam and I have an amazing bond, we're really great friends."
Meanwhile, Sam previously opened up to PopSugar, praising his co-star as "extremely talented and an extremely nice person."
"Caitríona and I get on very well, and I'm very lucky," he said, later adding: "We have a lot of fun."
Caitríona recently admitted that the co-stars don't see each other outside of work as often as they used to due to their "much more complicated and busy" lives.
While speaking at the 2023 Austin Television Festival, the 43-year-old explained that when the actors first began working together, she and Sam "were very similar in our personalities".
"We like to be very prepared and focused but then we like to have a lot of fun. We like to not take ourselves too seriously even though we take the work seriously," she said.
"But as our lives have gotten more complicated and busy, we may not see each other as much as we used to, but that core and that friendship and that base that we have has never changed."