"Outlander" star Sam Heughan is celebrating Caitriona Balfe's 39th birthday.

The 38-year-old actor dedicated a sweet and funny message to Balfe, his co-star on the Starz series, on her birthday Thursday.

"HAPPY BIIIIIIIIRTHDAY to the wonderful, funny, delightful and super talented @caitrionabalfe!!!" Heughan captioned a photo of the actress.

"You're amazing and I'm so lucky to get to work with you. Even if you ARE so much older than me (and equally childish in your sense of humor!"

"Have an amazing day and I know this year will be incredible for you. I hope all your dreams come true! #birthdayHugs," he added.

Heughan and Balfe play Jamie Fraser and Claire Fraiser on "Outlander," which will return for a fourth season in November.

The pair appear together on the Oct. 12 cover of Entertainment Weekly, and discussed the new season in the interview.

"Throughout the last three seasons, we've had them discover each other, go through this honeymoon period, and then try to establish themselves a home," Heughan said of Jamie and Claire.

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"This is an opportunity to really ground themselves and find a home together."

"Of course, it is Outlander so things don't run smoothly," he added. "But this season is really setting up the years to come. We're really establishing Fraser's Ridge and this new world."

"Outlander" is based on the Diana Gabaldon book series of the same name.

The series co-stars Laura Donnelly, Steven Cree, Richard Rankin and Sophie Skelton, and will premiere its fourth season Nov. 4.

Photo credit: Sam Heughan/Instagram