Polish Finance Minister Zyta Gilowska has been dismissed amid allegations that she covered up communist-era collaboration.
Mrs Gilowska, 56, was seen as a guarantor of fiscal responsibility
The prime minister accepted her resignation after a special prosecutor launched court proceedings against her.
In Poland, people who worked for the communist secret services can still hold public office, but it is an offence to lie about it.
Mrs Gilowska denied the allegations, saying she was a victim of blackmail.
Prime Minister Kazimierz Marcinkiewicz told a news conference he had asked her to stand down.
"The principles accepted by this government mean that her resignation must take place," he said.
He named his economic adviser, 46-year-old US-trained Pawel Wojciechowski, to replace her. In 1999-2005 he headed the Polish pension fund arm of the German insurer Allianz.
Following media reports, a special prosecutor said there was evidence to suggest Mrs Gilowska had lied in her 2001 declaration, in which she said she had never collaborated with the communist-era secret police.
The prosecutor, Wlodzimierz Olszewski, said "material gathered suggested a possibility that Ms Gilowska might have filed an untrue screening declaration."
Many, including the financial markets, saw her as a guarantor of fiscal responsibility in Poland's new conservative coalition, the BBC's Adam Easton reports from Warsaw.
Her offer to resign sparked a sell-off of Polish zlotys and treasury bonds, but the markets stabilised once her replacement was named.
Mrs Gilowska blamed the prosecutor's move against her on "lies, blackmail, false accusations" by political enemies.