The actor honored his longtime friend following her death on Monday after a battle with cancer.
Travolta, 68, took to Instagram Monday evening to share a pair of photos honoring Alley's memory -- including flashback photo of Alley in a white dress, and a photo of himself and Alley from their younger years.
"Kirstie was one of the most special relationships I’ve ever had," Travolta captioned the sweet tribute. "I love you Kirstie. I know we will see each other again."
Travolta and Alley first became friends when they played on-screen love interests in the 1989 comedy Look Who's Talking, and its sequels. They remained close friends throughout the years, and found a common connection through their involvement in the Church of Scientology.
According to a statement penned by Alley's children, True, 30 and Lillie, 28, and released on her official Twitter account, the actress died Monday following a short battle with cancer.
Per Alley's family, she was surrounded by her closest family as she fought the disease -- only recently discovered.
"To all our friends, far and wide around the world," the statement began. "We are sad to inform you that our incredible fierce and loving mother has passed away after a battle with cancer, only recently discovered.
"She was surrounded by her closest family and fought with great strength, leaving us with a certainty of her never-ending joy of living and whatever adventures lie ahead. As iconic as she was on-screen, she was an even more amazing mother and grandmother," the statement shared.