His career in politics was latterly overshadowed by a long-running sexual assault case.
Mr Lepper served in government as deputy prime minister to Jaroslaw Kaczynski in a 2006-2007 coalition.
Police spokesman Mariusz Sokolowski told the Polish news agency Pap: "Police are at the scene ... but all indications are that the politician committed suicide."
Mr Lepper's political career took off in the 1990s when he led stormy protests against government policies he regarded as damaging to farmers.
As leader of Self-Defence, he took the party to 11% of the vote in the 2005 general election.
He joined forces with Mr Kaczynski's Law and Justice Party, becoming both his deputy and minister for agriculture.
However, the conservative-populist coalition fell apart amid infighting, and Self-Defence's share of the vote plummeted to 1.5% in the next election, in 2007.
Mr Lepper, a former boxer and pig farmer, also ran four times for president of Poland.
'Hard man' Police were called to the party office by one of Mr Lepper's aides - the politician's body was found inside.
"We have ruled out the involvement of third parties," the police spokesman said.
A party colleague, Janusz Maksymiuk, told Polish television he had met Mr Lepper on Thursday and there was "no sign that anything was wrong".
"Lepper was a hard man," he added.
The sex scandal, which erupted in December 2006, shook Polish politics and contributed to the fall of the coalition.
Mr Lepper was convicted of soliciting sexual favours in exchange for job opportunities with his party but he avoided jail by appealing against the verdict, and was awarded a retrial this year.
Regarded by his supporters as a champion of the poor, he once promised to order the central bank to print huge quantities of cash to distribute to the needy, if he took power.
Another populist policy was advocating castration without pain-killers for convicted paedophiles.
His advocacy of a new style of economy situated somewhere between capitalism and socialism gained him some unusual friends, such as Belarusian President Alexander Lukashenko and Venezuela's Hugo Chavez, Polish radio reported.
Mr Lepper had also faced several criminal charges for acts of civil disobedience such as dumping grain on railway tracks and for defaming fellow politicians.