"Malaysia does not support the reimposition of the unilateral sanctions by the US against Iran," he told the Doha Forum, also attended by Qatari Emir Tamim bin Hamad Al-Thani, according to Reuters.
Malaysia and other countries have lost a "a big market" because of the sanctions on Iran, he said.
"Such sanctions clearly violate the United Nations Charter and international law; sanctions can only be applied by the United Nations in accordance with the charter," he added.
The US has ratcheted up pressure on Iran since last year after withdrawing from the 2015 nuclear deal, known as the Joint Comprehensive Plan of Action (JCPOA).
A first round of American sanctions took effect in August last year, targeting Iran's access to the US dollar, metals trading, coal, industrial software, and auto sector. A second round was imposed on November 4, targeting Iran's energy sector and financial transactions.