"My dearest Olivia, you made all of our lives so much better," he captioned a throwback photo of Newton-John. "Your impact was incredible. I love you so much. We will see you down the road and we will all be together again."
"Yours from the moment I saw you and forever! Your Danny, your John!" he concluded.
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Newton-John died Monday morning "at her Ranch in Southern California," where she was "surrounded by family and friends," read the announcement alongside a stunning photo of the actress and singer-songwriter on her official Instagram Monday.
"We ask that everyone please respect the family's privacy during this very difficult time," the caption, attributed to Newton-John's husband John Easterling, continued.
Easterling, 70, went on to recognize Newton-John as "a symbol of triumphs and hope for over 30 years sharing her journey with breast cancer.
Alberto E. Rodriguez/Getty Images John Travolta and Olivia Newton-John in 2018
"Her healing inspiration and pioneering experience with plant medicine continues with the Olivia Newton-John Foundation Fund, dedicated to researching plant medicine and cancer," he added.
"In lieu of flowers, the family asks that any donations be made in her memory to the @onjfoundation."
"Olivia is survived by her husband John Easterling; daughter Chloe Lattanzi; sister Sarah Newton-John; brother Toby Newton-John; nieces and nephews Tottie, Fiona and Brett Goldsmith; Emerson, Charlie, Zac, Jeremy, Randall, and Pierz Newton-John; Jude Newton-Stock, Layla Lee; Kira and Tasha Edelstein; and Brin and Valerie Hall," Easterling concluded,
RELATED VIDEO: John Travolta Says He's "Very Proud" of Grease Costar Olivia Newton-John as She Faces Cancer
Travolta — whose wife Kelly Preston also died of breast cancer, in 2020 — and Newton-John remained close over the years since starring in 1978's Grease as iconic lovebirds Danny and Sandy, two teens who couldn't be more different but form an unlikely romance.
The duo even revisited their characters for the first time in more than 40 years in December 2019, dressing up as Danny and Sandy during the Meet n' Grease sing-along event at the Coral Sky Amphitheater in West Palm Beach, Florida.
Travolta has also been vocal about his support for Newton-John as she battled cancer, telling Entertainment Tonight in August of that year, "She looks incredible."
He added, "She doesn't look any different than [she did] years ago, and I'm very proud of her."
In PEOPLE's Grease! 40th-anniversary special edition issue the previous year, Travolta and Newton-John discussed their enduring friendship since making the classic musical film.
"When you share that kind of meteoric success — and nothing has been able to exceed it — you share a bond," said Travolta, adding that he still texted with Newton-John. "I've been through her having a child, getting divorced, losing her sister. She's been through my getting married, having children. It's wonderful and full of shared memories."
"We did something life-changing, making that film," Newton-John remarked. "[At the premiere] you got the feeling from the energy that something was happening. It was a huge response."
She added of Travolta, "I feel grateful to be a part of that and to have worked with him. We've stayed friends ever since."