Catherine's husband and Kirk's son, Michael Douglas, also paid tribute with a heartfelt video he shared to Instagram early Tuesday morning.
"Happy Birthday Dad! I miss you and thank you for always giving the best advice! Love, Michael," the proud son captioned the post.
The clip featured a montage of photos of the father and son duo from across the decades, and included a voice over from Michael revealing the best bit of advice he ever got from his famous father.
"The best advice my father gave me was, 'Whatever you do, you do it to the best of your ability. You leave that job knowing that you couldn't have done any more, and then walk away, you've done the best. Don't look back.'"
"There wasn't anyone like him. There's another generation, that was the great generation, post-World War II…and Dad was one of kind," Michael shared. "
"I think he's very happy. I know he takes particular pride -- not so much in whatever I worked with and I'm doing with the Motion Picture Television Fund -- but I know he loves Catherine [and] the work that she's doing."
"He always used to tease me. We would call him up and he would be like, 'Enough with you, let me talk to your wife,'" Michael cracks. "He was a flirt until the very end."