"This is a hostile classification which lacks legitimacy, because Muthanna Al-Dhari adheres to ethical standards in the struggle against American occupiers," the movement's official spokesman Jamil Abu Bakr said in a statement.
Muthanna, who visits Jordan frequently, is son of Hareth Al-Dhari, head of the Iraqi Scholars Association, which is campaigning to ensure a withdrawal of the US-led military coalition.
According to local reports, Foreign Minister Nasser Judeh has notified the Finance Ministry and other major financial institutions to place Al-Dhari on the Unified Terrorism List in compliance with instructions from the UN Security Council.
"We don't see any justification for the government's capitulation to such international pressures which are being exercised by proxy on behalf of hostile powers," Abu Bakr said.
He voiced hope that the government would reconsider its decision. "Al-Dhari and his likes deserve backing because they fight for a just cause," he added.
The Muslim Brotherhood is Jordan's major opposition political gathering, sympathetic of Iraqi groups, which seek an eviction of the US forces from the neighboring country.
Jordan currently hosts about 500,000 Iraqis who fled the country for safety after the 2003 invasion.
