By Dave Michaels

A former trader at Bank of New York Mellon Corp. who alerted authorities to the bank's pattern of overcharging big clients on currency trades was awarded a $50 million whistleblower payment Thursday.

The award, the largest of its type made by the Securities and Exchange Commission, comes more than a decade after the trader, Grant Wilson, began assisting authorities with the currency-trading investigations, according to a person familiar with the matter. The bank paid $714 million in fines and other compensation in 2015 to resolve allegations it defrauded pension funds and other clients related to currency transactions.

The investigations were among the broadest enforcement efforts ever against banks that trade in global currencies -- one of the world's biggest financial marketplaces. Large pension funds that invested in global assets on behalf of teachers, police and other retirees learned they were given close to the least-favorable exchange rates of the day, with the bank profiting from the difference.

The SEC didn't name Mr. Wilson or the bank in its press release or an order documenting the award, but Mr. Wilson is the whistleblower whose information was crucial for moving the case forward, the person said. Mr. Wilson assisted the government for two years while working as a currency trader at BNY Mellon, The Wall Street Journal reported in a 2011 article. He left the bank that year, the Journal reported.

Mr. Wilson and a bank spokesman couldn't immediately be reached for comment.

A spokesman for the SEC, which doesn't reveal information about whistleblowers' identities, declined to comment.

Mr. Wilson provided extensive documentation, including his descriptions of how the bank processed trades that resulted in client overcharges. BNY Mellon told customers that its service was "free of charge" and designed to help clients "minimize risks and costs."

The bank said when it settled the investigations in 2015 that it was "pleased to put these legacy FX matters behind us, which is in the best interest of our company and our constituents." The settlement with the SEC required the bank to report on compliance improvements that it planned to implement over a period of three years. It also had made changes to its pricing disclosures in 2012.

The whistleblower's assistance to U.S. officials involved "firsthand observations of misconduct" and details that "provided a road map for the investigation," the SEC said in an order announcing the award.

"Whistleblowers have proven to be a critical tool in the enforcement arsenal to combat fraud and protect investors," Jane Norberg, the chief of the SEC's whistleblower office, said in a news release.

The SEC's whistleblower program, which has grown in prominence over recent years, allows people who provide original leads about illegal conduct, or give information that significantly advances an open investigation, to keep between 10% and 30% of the total fines collected in a case. The SEC's settlement with BNY Mellon, detailed in 2016, ordered the bank to pay about $163 million in fines. The agency has awarded over $500 million to whistleblowers over the nine-year life of the program, officials said Thursday.

Federal prosecutors in Manhattan and state authorities also investigated BNY Mellon over the foreign-exchange misconduct. The $714 million included fines paid to the U.S. Department of Justice and New York state, and payments to customers who filed a class-action lawsuit.

Write to Dave Michaels at dave.michaels@wsj.com