Sam Heughan, 39, and Caitriona Balfe, 40, share a very close friendship to the point that they were rumored to be dating. Their effective portrayal as Jamie Fraser and Claire Randall in the “Outlander” series did not help as it just intensified the romance rumors. Recently, the Scottish actor proved how their friendship has grown after years of working together through a sweet birthday tribute for his on-screen ladylove.

On Thursday, Oct. 4, Balfe celebrated her natal day with an outpouring love from her family and close pals. Among those who greeted the former model is Heughan. The "Outlander" Season 5 actor shared a photo of them on his official Instagram account. In the snap, Heughan looked dapper in his dark blue-colored outfit while the “Outlander” Season 5 actress donned an orange-colored dress.

“Happy Birthday @caitrionabalfe !!! (...This is the only non inappropriate picture I could find! _ ) Wishing this remarkable, generous, talented, gracious, elegant, big hearted, goof, the biggest birthday wishes!!,” Sam Heughan wrote in the caption. “This is only the start of your success..Love ya Screen Wife! _ _ x."

Caitriona Balfe Sam Heughan
Caitriona Balfe was nominated for a fourth time at the Golden Globes. Balfe and Sam Heughan are pictured at SCAD Savannah Film Festival on Oct. 28, 2018 in Savannah, Georgia. Cindy Ord/Getty Images for SCAD

Balfe was quick to respond to Heughan’s sweet greeting. “Awwwww right back at you darling! I’ll hold the fort down on 40 till you get here in a few months!” she wrote. Just a few hours before her red-letter day, the fiancée of Tony McGill shared a photo of her co-stars on the same social media platform. The black-and-white snap, which was taken while filming “Outlander” Season 5, featured the pair alongside Sophie Skelton.

Aside from Heughan, Skelton, Cesar Domboy, and Lauren Lyle also greeted Balfe through social media. Skelton, who plays the role of Brianna in the popular drama television series, wished the actress well as she and Heughan are set to promote the upcoming premiere of “Outlander” Season 5 at the New York Comic-Con 2019.

During the 2019 Toronto International Film Festival in September, Caitriona Balfe revealed to Entertainment Tonight that the forthcoming new installment is “really, really good.” The on-screen ladylove of Sam Heughan even teased that the Fraser clan could stay in the past in “Outlander” Season 5, which is scheduled to premiere on Feb. 16, 2020.