SINGAPORE—Singapore authorities said on Friday they plan to sanctionGoldman Sachs Group’s former top executive in Southeast Asia after the banker allegedly wrote an unauthorized reference letter for a Malaysian who U.S. authorities believe is at the center of one of the world’s largest-ever financial scandals.

Goldman put the banker, Tim Leissner, its former chairman of Southeast Asia, on leave at the start of this year after it discovered he had written the reference letter on behalf of Jho Low, the young Malaysian financier who U.S. authorities have named as a key figure in the scandal revolving around Malaysian state investment fund 1MDB.

The Monetary Authority of Singapore, in a statement, said it had decided to ban Mr. Leissner from working in the city state’s financial markets for 10 years. It said the former Goldman banker had written the letter to a Luxembourg financial institution claiming that Goldman had done due diligence on Mr. Low and had not found any concerns of money laundering. “These statements were untrue and were made by Mr. Leissner without Goldman Sachs’ knowledge or consent,” the statement said.

The U.S. Department of Justice in a lawsuit earlier this year alleged that more than $3 billion had been siphoned out of the fund, 1Malaysia Development Bhd., and gone into real estate, Hollywood movies and other assets. The suit said Mr. Low had benefited from some of the stolen money. The 1MDB fund has denied any wrongdoing.

The MAS also said it had levied million-dollar fines on the local branches ofStandard Chartered Ltd. and private bank Coutts & Co Ltd. for their handling of accounts related to the 1MDB scandal. It fined Standard Chartered S$5.2 million ($3.6 million), and imposed a fine of S$2.4 million on Coutts.

Attempts to reach Mr. Leissner, who is living in Los Angeles, were not immediately successful. He has denied wrongdoing. Mr. Leissner was not questioned by Singapore authorities over the matter, according to a person familiar with the matter.

Goldman Sachs made around $600 million in 2012 and 2013 in fees, selling a total of $6.5 billion in bonds for 1MDB, among its largest paydays anywhere in the world in those years. Much of the proceeds of the bonds were siphoned off immediately into offshore accounts, the U.S. Department of Justice said in a lawsuit this summer.

The bank said in a statement Friday that the MAS action against Mr. Leissner was related to a matter discovered in January 2016 that it reported to Singapore authorities. The bank added that it is cooperating with authorities.

Attempts to reach Mr. Low, whose whereabouts is unknown, were unsuccessful.

Standard Chartered said in a statement it regrets that 1MDB-related transactions passed through the bank. It said it had reported the suspicious transactions, cooperated with authorities and had taken action to strengthen its controls.

A spokesperson for Coutts could not immediately be reached for comment.