The unnamed portrait was among 18 paintings stolen 16 years ago from the home of the late English art collector and philanthropist Sir Alfred Beit, at Russborough House, south of Dublin .
Detectives from the police Arts and Antiques Unit found the Rubens in north Dublin on Tuesday, after acting on intelligence.
No one was arrested and a police spokeswoman declined to give exact details of where the painting was found for operational reasons.
Police said the recovered Rubens portrait had been returned to the Alfred Beit Foundation.
It was set up in 1976 to give financial assistance to many arts organisations and to award scholarships in the fields of visual arts, music, and dance.
Beit, whose family wealth came from gold mines and diamond dealing in South Africa, inherited a collection of Old Masters from his father.
He decided to house them in Russborough House after buying and restoring it in 1952. He died in 1994.
After the 1986 robbery, carried out by a 13-member gang led by Dublin gangster Martin Cahill, much of the haul was gradually recovered, largely by auction houses in England and Belgium.
Only two paintings are still missing.
Cahill, who was killed in 1994 by the IRA, has been the subject of two Hollywood films featuring the art heist - Ordinary Decent Criminal starring Kevin Spacey and The General directed by John Boorman.
This was not the first time the mansion had been targeted by criminal gangs - it has had artworks stolen three times in total.
In 1974 an Irish Republican Army gang stole 19 paintings after tying up Sir Alfred and his wife.
All the paintings were later found in County Cork in southwest Ireland.
And in June 2001 two more paintings worth £2.3m were stolen.