In a speech from Elizabeth Palace, Michael acknowledged that supporting a relative for public office was not a "normal thing for a member of the royal household," but said he did so because "Romania is going through hard times" during the global economic crisis, including an emergency loan from the International Monetary Fund.
Then again, the former king's life has been filled with unusual moves.
Michael is a great-great-grandson of Britain's Queen Victoria and a third cousin of Queen Elizabeth II.
At 87, he is one of the last surviving heads of state from World War II.
He reigned as king of the Romanians as a boy from 1927 to 1930, and again from 1940 until he was forced to abdicate on 1947 by the communists backed by Soviet ruler Josef Stalin.
Communism ended in Romania in 1989, but it was only in recent years that authorities have made concessions to the king and his family by allowing them to return home from exile in Switzerland and to use property such as the palace that they once owned.
Romania hasn't scheduled its next presidential election yet, but it is expected by the end of this year.
Prince Radu Duda - a former Romanian actor married to Michael's eldest daughter, Princess Margaret - is so far the only declared candidate. But President Traian Basecsu is expected to run for re-election and, if so, would probably be the favorite.
In announcing the endorsement of Duda, former King Michael said: "I always did things well for the good of the country. I never forced things, and I never benefited personally from the changes in recent history. .... I call on all of you to support this important step for Romania."
Michael is popular with some Romanians, but many in the younger generation know little about the roles he played in as the country's monarch. For that reason, Duda may not be a favorite in the presidential election, even with his father-in-law's endorsement.
Whatever happens, it won't be the former king's first brush with politics.
In 1992, when he briefly returned to Romania from exile to celebrate Orthodox Easter he was cheered by tens of thousands of people and invited by the main opposition party to run for president. He turned down the offer.