"Happy Birthday you," she wrote along with a cake emoji.
Along with the Insta message, Alia uploaded a photograph of Ranbir from their recent trip to Kenya.
In the image, Ranbir is seen wearing a black t-shirt with his reflective sunglasses hanging from his neck, wearing a hat and holding a binocular in his hand.
On the work front, Alia and Ranbir unite on screen in Ayan Mukerji's "Brahmastra". The film, touted to be one Bollywood's most expensive ever, is an action fantasy adventure that also stars Amitabh Bachchan, Dimple Kapadia, Mouni Roy and Telugu superstar Nagarjuna. "Brahmastra" is being billed as the first of a three-part series.
While "Brahmastra" is scheduled to release in 2020, Ranbir has another release lined up next year. He will be seen playing a dacoit in the Yash Raj Films-produced pre-Independence period drama "Shamshera". The film is directed by Karan Malhotra and co-stars Vaani Kapoor.
Alia, who was recently seen in the box-office disappointment "Kalank", returns next year in "Sadak 2", which marks her father Mahesh Bhatt's return to direction after over two decades.
Alia also has "Bahubali" maker S. Rajamouli's next film, "RRR", coming up next year. The film stars Ajay Devgn, as well as the Telugu stars NTR Junior and Ram Charan Teja.
( With inputs from IANS )