The collision happened near High Valleyfield on the A985 road, at a junction close to the B9037, at about 3:45pm on Friday April 19.
In a statement released today, police in Fife said a 54-year-old man driving a green Ford Escort westbound was involved in a collision with a silver Renault Megane being driven by the pensioner, who was approaching the A985 from the Culross direction.
The 84-year-old was taken to Ninewells Hospital in Dundee but died from his injuries on Sunday, April 28.
The 54-year-old was taken to Edinburgh Royal Infirmary by air ambulance with serious injuries.
Police are appealing for witnesses following the crash, which resulted in the road being closed for five and a half hours as crash scene investigations took place.
Sergeant Stephen Quinn, of Fife’s Road Policing Unit, said: “Sadly, as a result of the collision, a man has lost his life and our thoughts are with his family and friends at this difficult time.
"The driver of the Ford Escort sustained serious injuries that he continues to recover from and we are grateful to those who stopped to assist at the scene of the collision and who have provided information to help with our inquiries so far.
"I’d ask anyone who has relevant dash-cam footage or information, and who hasn’t yet provided this to officers, to contact us as soon as possible."
Those with information should contact Police Scotland on 101, quoting incident number 2852 of 19 April.