The singer, actor and philanthropist, died on August 8 after a long struggle with breast cancer.
Travolta, 68, shared a picture of himself and his former co-star together in their hit film Grease on Instagram, and captioning the image: "Happy Birthday my Olivia."
Newton-John rose to fame after starring as Sandra Dee in the hit 1978 musical, opposite Travolta as Danny Zukko.
She died at the age of 73 after a long battle with breast cancer.
"My dearest Olivia, you made all of our lives so much better," Travolta wrote."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 first moment I saw you and forever! Your Danny, your John!"