"I am deeply saddened to learn of the passing away of your father Shri Rajesh Khanna," he wrote.
"In the passing away of the Shri Rajesh Khanna, the Indian film industry has lost one of its most successful and most charming heroes," he said.
Observing that Khanna was a celebrated artiste who entertained millions of Indians with his performance in a variety of films, Singh said, "his legacy will live on in the form of the numerous entertaining and acclaimed films that he leaves behind."
The Prime Minister noted that Khanna was called the first superstar of the Indian cinema and the powerful roles he essayed in classics like Anand, Aradhana, Kati Patang and Amar Prem stand testimony to his artistic genius.
"I convey my heartfelt condolences to the members of the bereaved family and Shri Khanna's countless fans and admirers. I pray for peace of the departed soul," Singh said.
He noted that Khanna was also a colleague in the Congress party and a member of the Lok Sabha in the early nineties.