"I will support and protect the constitution of the Philippines," pledged the former first lady whose name has become synonymous with massive official corruption.
Marcos, who fled with her husband Ferdinand to Hawaii after he was overthrown as president in a February 1986 popular revolt, also promised "to respect and abide" by Philippine laws.
Marcos still faces an 18-year jail term imposed when she was convicted on corruption charges arising from her husband's 20 years in power, nine of which the country spent under martial law after Ferdinand Marcos suspended constitutionally guaranteed civil liberties.
The eccentric 66-year-old Imelda Marcos is currently appealing the jail term and would automatically forfeit her House seat if the appeal is rejected. However, the appeal is likely to take years to work its way through the Philippines' labyrinthine legal system.
The Marcos family and their associates stand accused of looting the Philippines of more than $5 billion, although Imelda Marcos vehemently denies the allegations.
In her oath, administered by Chief Justice Andres Narvasa, she said she would enter into its undertakings "without any hesitation, or any plan to renege, so help me God."
Narvasa declined to speak to reporters after the brief ceremony, giving Marcos only a curt "Congratulations" before leaving.
"My only obsession is to help the people," Marcos later told reporters on the steps of the Supreme Court.
It was the Supreme Court which cleared the way Wednesday for Marcos to take up the seat she won in national elections in May representing the First District of her native Leyte province in the central Philippines.
Marcos won by a large 70,000 votes but her victory was first challenged by the watchdog Commission on Elections and then by Cirilo Montejo, the incumbent congressman she defeated.
The court rejected Montejo's challenge earlier this week and Marcos immediately announced this was God's will.
"I know that the voice of the people is the voice of God and if you are on the side of God, who can be against you?" she said.
Marcos was allowed to return to the Philippines after her husband died in 1989. She lost in a bid for the presidency in 1992 elections.
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