The Rosemary's Baby actress was married to the legendary crooner for two years from 1966 but they allegedly remained occasional lovers for years afterwards.
Critics have long speculated that her son Ronan, now 25, was fathered by Sinatra and not moviemaker Woody Allen, who Farrow was married to at the time of the child's birth.
Now Farrow has spoken out about the long-running rumour and admitted it might be true.
Asked by a journalist at Vanity Fair magazine if Sinatra is Ronan's father, she replies, "Possibly."
Ronan, a journalist and human rights activist, reportedly disowned Allen after he split from Farrow and began a relationship with her adopted daughter Soon-Yi Previn.
Sinatra died following a heart attack in 1998.