Canadian scientist Ralph Steinman will keep his Nobel prize for medicine, the Nobel Foundation has said, after his death on Friday threw it into doubt.
The rules state "work by a person since deceased shall not be considered".
But the foundation said it was unaware of Prof Steinman's death from pancreatic cancer and that the award had been made "in good faith".
Prof Steinman was announced the joint winner on Monday, along with Prof Bruce Beutler and Prof Jules Hoffman.
Prof Steinman received the award for revolutionising the understanding of the immune system.
He discovered the dendritic cell, which helps the body fight off infection.
A statement on the Rockefeller University website, where Prof Steinman worked, said the scientist had died on 30 September at the age of 68.
He was diagnosed with pancreatic cancer four years ago.
The foundation said in a statement: "The events that have occurred are unique and, to the best of our knowledge, are unprecedented in the history of the Nobel Prize.
"According to the statutes of the Nobel Foundation, work produced by a person since deceased shall not be given an award. However, the statutes specify that if a person has been awarded a prize and has died before receiving it, the prize may be presented.
"An interpretation of the purpose of this rule leads to the conclusion that Ralph Steinman shall be awarded the 2011 Nobel Prize in Physiology or Medicine."
It said the award had been made "in good faith, based on the assumption that the Nobel Laureate was alive. This was true - though not at the time of the decision - only a day or so previously".
Ahead of the foundation's statement, Rockefeller University president Marc Tessier-Lavigne said it was "delighted that the Nobel Foundation [had] recognised Ralph Steinman for his seminal discoveries concerning the body's immune responses.
"But the news is bittersweet, as we also learned this morning from Ralph's family that he passed a few days ago after a long battle with cancer.
"Our thoughts are with Ralph's wife, children and family."
Prof Steinman was being treated with a dendritic-cell based therapy which he designed himself.
The Nobel Foundation expressed "deep sadness and regret" and said that "our thoughts are with Ralph Steinman's family and colleagues".
In 1931, Erik Axel Karlfeldt was posthumously awarded the Nobel prize for literature, but the rules were changed in 1974.
Three pioneers of the immune system shared this year's award for medicine.
Prof Beutler and Prof Hoffman discovered how the body's first line of defence was activated.
The Nobel prize committee said: "Together Bruce Beutler, Jules Hoffmann and Ralph Steinman have revolutionised understanding of the immune system by discovering key principles for its activation.
"They have opened up new avenues for prevention and therapy."