The president's death could lead to a sympathy vote for his brother Jaroslaw
The twin brother of Poland's former president, who was killed in a plane crash in Russia this month, says he will run to be elected in his place.
Jaroslaw Kaczynski said he would stand in the 20 June presidential election despite his "personal pain", in order to continue his brother's mission.
Lech Kaczynski was killed in a plane crash in western Russia on 10 April, along with many Polish dignitaries.
The two brothers founded the Law and Justice party in 2001.
Possible sympathy vote
In a statement on the party's website on Monday, Mr Kaczynski said: "Poland is our common, great responsibility. It demands that we overcome personal suffering to take action despite a personal tragedy."
He continued: "This is why I have taken the decision to be a candidate for the office of president of the Republic of Poland."
Jaroslaw Kaczynski, 60, became prime minister in 2006, heading a right-wing coalition that had tense relations with the European Union and Russia.
He lost power in a general election in 2007. Lech Kaczynski had been expected to run for re-election in October - although polls suggested he would lose.
The BBC's Adam Easton in Warsaw says his brother could benefit from a significant sympathy vote in the wake of the tragedy.
Parliamentary Speaker Bronislaw Komorowski, who became acting head of state after the crash, had been preparing to run against Lech Kaczynski.
Opinion polls have suggested that Mr Komorowski will defeat Mr Kaczynski in the snap election on June 20.
The election was called after the president, his wife and 94 senior officials were killed in a plane crash in Russia on 10 April.
Kaczynski attended the funeral on Monday of Aleksandra Natalli-Swiat, a member of his party, at a cemetery in Wroclaw. Natalli-Swiat was one of the victims of the plane crash.