Arthur Miller, pictured here with Marilyn Monroe, hid his son away for 40 years
Miller, whose plays examined questions of guilt and morality, virtually cut the boy out of his life after committing him to a mental institution when he was one week old.
The secret son, named Daniel, now nearly 41, did not receive a mention in his father's memoir, Timebends.
But Miller, author of Death Of A Salesman and The Crucible, finally relented just six weeks before he died two years ago, according to an article in Vanity Fair magazine.
He then added him to his will, giving him an equal share of his estate, along with his other three children.
Daniel is the younger brother of Rebecca Miller, the actress wife of Daniel Day-Lewis. Day-Lewis, who won an Oscar for his role as a disabled person in My Left Foot, was said to be "appalled" at the way his brother-in-law was treated and may have pressed Miller to make amends.
Miller's third wife, photographer Inge Morath, who he met when she was taking pictures of Monroe on the set of The Misfits, gave birth to the boy in 1966.
Miller apparently called his son "a mongoloid" and told a friend: "I'm going to have to put the baby away."
Although his wife wanted to keep the child, Miller refused because he didn't want Rebecca to grow up with him.
(<-------- Inge Morath, Miller's 3rd wife, Daniel's mom)
Daniel Miller is said to have lived in an overcrowded Connecticut institution that was sued in the 1980s over its poor living conditions.
He left at 17 and went on to compete in the Special Olympics in skiing, cycling, track and bowls.
He is now living with an elderly couple. Rebecca Miller has insisted that her brother "is very much part of our family". She added: "He leads a very active, happy life surrounded by people who love him."
But the picture that emerges in the piece is of a father living in denial. He almost never mentioned his son to friends and hardly ever joined his wife on her weekly visits to see him.
"Miller excised a central character who didn't fit the plot of his life as he wanted it," said writer Suzanna Andrews.
The revelation is at odds with Miller's reputation as a champion of the downtrodden and a hero of the Left.
In the days after his death, aged 89, in 2005, Miller was eulogised around the world. The New York Times extolled his "fierce belief in man's responsibility to his fellow man" and fellow American playwright Edward Albee said Miller had held up a mirror and told society: "Here is how you behave."
He was also lauded for refusing to give up names of communist sympathisers in the American red purges in the 1950s and for his outspoken opposition to the Vietnam War.
These new disclosures are certain to spark a re-examination of his life and works. A fierce debate has already broken out on the internet, raising questions about Miller's legacy.
One scathing blog suggests the story "ought to damage permanently Miller's reputation, if not as a writer, then as a humanitarian."
His best plays were written before Daniel was born. The Vanity Fair story suggests Miller's guilt may have overshadowed his subsequent career.
"Whether he was motivated by shame, selfishness or fear - or, most likely, all three - Miller's failure to tackle the truth created a hole in the heart of his story," wrote Miss Andrews.
"One wonders if, in his relationship with Daniel, Miller was sitting on his greatest unwritten play."
Daniel's existence was said to have been an open secret among Miller's coterie. The story was first mentioned in a 2003 biography by American theatre critic Martin Gottfried, who said yesterday that he thought Miller's reputation as a "great, great" playwright is secure.