"To the world he was a legend, an actor from the golden age of movies who lived well into his golden years, a humanitarian whose commitment to justice and the causes he believed in set a standard for all of us to aspire to," Michael Douglas wrote. "But to me and my brothers Joel and Peter he was simply Dad, to Catherine, a wonderful father-in-law, to his grandchildren and great grandchild their loving grandfather, and to his wife Anne, a wonderful husband."
Michael and his wife of 20 years, Catherine Zeta-Jones, have remained faithful children to the late Kirk Douglas, regularly taking time to thank him during historic moments of their careers.
In 2018, when Michael was honored with a star on the Hollywood Walk of Fame, he was sure to recognize his father in attendance.
"It means so much to me, Dad, that you're here today," he said while tearing up. "I thank you for your advice, for your inspiration, and I'll say it simply and with all my heart: I'm so proud to be your son."
Zeta-Jones and Kirk Douglas presented the Golden Globe for Best Screenplay together in January 2018 and he was honored with a standing ovation by the audience.
Zeta-Jones noted her then-101-year-old father-in-law's history of success on-screen and how he helped break Hollywood's blacklist during the Cold War.
"I have something to say but I could never follow you," Kirk Douglas joked at the time.
On his 103rd birthday, Zeta-Jones posted a birthday message to the Hollywood legend.
"I love you with all my heart," she wrote.
On Wednesday, Michael echoed those sentiments in his tribute.
"Kirk's life was well lived, and he leaves a legacy in film that will endure for generations to come, and a history as a renowned philanthropist who worked to aid the public and bring peace to the planet," he said. "Let me end with the words I told him on his last birthday and which will always remain true. Dad- I love you so much and I am so proud to be your son."
Later on Wednesday, Zeta-Jones posted a black-and-white photo of herself kissing Kirk.
"To my darling Kirk, I shall love you for the rest of my life. I miss you already. Sleep tight..." she said.
Wednesday evening, other celebrities began to pay their respects to the late actor.
"In front of, or behind the camera, the significance and magnificence of Kirk Douglas cannot be overstated," chef and TV personality Alton Brown wrote.
"The SAG Awards mourns the loss of the legendary and talented Life Achievement Recipient Kirk Douglas. His commitment to acting and justice were inspirational," the official Screen Actors Guild Awards account tweeted. "Our thoughts go out to his family at this time."
"I will always be in awe. May your memory forever be a blessing," Ed Asner posted.
"Bravo Kirk Douglas on an incredible life. Thank you for so generously sharing your amazing talent with all of us," actress Mitzi Gaynor posted. She starred in the 1963 film "For Love or Money."
"The film we made together will always hold a special place in my heart," she wrote. "My love to Anne & your beautiful family."