Preston married the Grease star on September 5, 1991, when she was two months pregnant with their first child Jett. They married a second time on September 12 in Florida.
Jett, born in April 1992, died at age 16 in January 2009.
The couple are also parents to daughter Ella, 20, and son Benjamin, 9.
In his post, Travolta wrote "Happy birthday hon! I found this photo of my mom and dad's wedding. It was nice to see ours alongside theirs. All my love, John."
Travolta previously shared the news of her death on Instagram on July 12, writing, "It is with a very heavy heart that I inform you that my beautiful wife Kelly has lost her two-year battle with breast cancer.
"She found a courageous fight with the love and support of so many. My family and I will forever be grateful to her doctors and nurses at MD Anderson Cancer Center, all the medical centers that have helped, as well as her many friends and loved ones who have been by her side.
"Kelly's love and life will always be remembered. I will be taking some time to be there for my children who have lost their mother, so forgive me in advance if you don't hear from us for a while. But please know that I will feel your outpouring of love in the weeks and months ahead as we heal. All my love, JT."
Preston's last post on Instagram was a Father's Day tribute to Travolta, posting a snap of her with him and their kids Ella and Ben as well as a photo of him with their late son Jett.
"Happy Father's Day to the best one I know, we love you," she wrote.
