Sam, who plays Jamie Fraser in the historical drama, took to Instagram to wish his fellow Men In Kilts presenter a happy birthday with a series of photos and heartfelt message.
WATCH: Sam Heughan and Graham McTavish are hugely popular on their show Men In Kilts
Sam's post sees the pair together looking suave in one photo, while the others demonstrate the banter and bond they share while working together both in front and behind the camera.
The caption read: "Happy birthday to the big man! My partner in crime, latte loving, map reading, snack pilfering, delightfully ridiculous, co-writer, co-producer and all round top (grey) dog! @grahammctavish We celebrate soon buddy!x."
It's safe to say that Sam's followers were delighted to see the pair's bond shown in the pictures, and flooded the comments section to express their love for the two actors.
Sam wished Graham a happy birthday on Instagram
One person said: "What I would do to spend a day with these two! Would be full of awe and laughter," as a second wrote: "Can't wait for you both to be together and into fun things for us to see."
A third echoed the praise, commenting: "You two have the best chemistry!", as a fourth added: "They are the best!!!"
Sam and Graham have shared a close friendship since both starring in Outlander together. However, since Graham moved on from the show, the pair have continued to work together in other ways.
The two are now the stars of their own adventure docu-series Men In Kilts, which sees the two Scots explore their homeland for the Starz TV show. For their upcoming new series, Sam and Graham will be heading to New Zealand.
Their success has also meant they've collaborated for a Men In Kilts spin-off book, The Clanlands Almanac, which is a New York Times best-seller.
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