Travolta posted a short but heartfelt message on Instagram saying: "My dearest Jetty, I miss you more than words can say. I think about you everyday. Happy Birthday. Love, your Dad."
Jett died in 2009 in the Bahamas after hitting his head on a bath while having a seizure. He was autistic, suffered from seizures and had a rare condition called Kawasaki Syndrome, where the wall of the arteries become inflamed.
In 2014 Travolta told US Weekly he didn't know if he would make it through the grief of losing his son.
"Life was no longer interesting to me, so it took a lot to get me better," he told US Weekly.
Jett was one of three children Travolta had with his late wife Kelly Preston, who died in 2020 at the age of 57 following a long battle with cancer.
Their daughter Ella commented on the post: "Happy Birthday Jetty, we love you so much.