Oscar winner Winslet, 33, and DiCaprio, 34, who made their international names in the 1997 film about the sinking of the liner were approached by a charity to help 97-year-old Millvina Dean.
The film’s director James Cameron has also made a donation.
Ms Dean from the New Forest, Hampshire, was just nine weeks old when the liner sank with the loss of more than 1,500 lives in April 1912.
Now frail, she has been living at a nursing home for a few years and last month she auctioned personal belongings and memorabilia associated with the disaster to help pay for the €3,340-a-month cost of her care.
The campaign called the Millvina Fund was launched in Belfast yesterday, where the liner was built.
It was launched in the former Harland & Wolff drawing offices in the Titanic Quarter of the city.
Irish author Don Mullan, a friend of Ms Dean, approached the stars to lend their support.
He has also produced a limited edition photograph depicting Millvina signing a card for an autograph collector, which are selling for €500 with all proceeds going towards the Millvina Fund.
In a joint statement, the acting stars said: “We are honoured to contribute to the Millvina Fund. Our hope is that others will feel inspired by Millvina Dean’s remarkable story of survival, and hope she can rest easier in knowing that her future will become more secure through this fund.”
It is believed that the Titanic trio of DiCaprio, Winslet and Cameron have donated €22,000. Mr Mullan said: “I laid down the challenge to the Titanic actors and directors to support the Millvina Fund and I was delighted with the generosity they have shown in meeting that challenge.
“To date, 30 copies of the print have been sold and I am confident the support of stars such as Kate Winslet and Leonardo DiCaprio will help boost interest in the prints and ensure they will sell. I am also determined to enlist the support of Celine Dion, who sang the theme tune from Titanic.”
Millvina Dean was the Titanic’s youngest passenger. Her father, Bertram Dean, died in the 1912 tragedy after saving the lives of his wife and two children. Millvina’s mother returned from New York with her children to Southampton, where Millvina has spent most of her life.
Una Reilly of the Belfast Titanic Society which helped set up the fund, added: “As the last living survivor from the most famous ship in the world, Millvina Dean has a very special place in the annals of Northern Ireland’s history and, indeed, across the globe.”