A top employee at BitMEX has pleaded guilty in New York to failing to put in place an anti-money-laundering program at the cryptocurrency derivatives exchange, joining three co-founders who previously admitted to violations of U.S. law.

Gregory Dwyer entered a guilty plea Monday in New York federal court, admitting to one count of violating the Bank Secrecy Act.

Prosecutors said Mr. Dwyer, one of the first employees of BitMEX and its onetime head of business development, was involved in BitMEX’s flouting of U.S. anti-money-laundering rules.

The Seychelles-incorporated exchange in 2015 announced a withdrawal from the U.S. market but made only minimal, ineffective efforts to stop U.S.-based customers from signing up to BitMEX, according to prosecutors. Thousands of U.S.-based clients were able to use the service even though they were meant to be blocked, and the controls put in place were “facade,” prosecutors said.

Prosecutors said BitMEX in effect operated as a money-laundering platform because it had no anti-money-laundering and know-your-customer programs.

“Today’s plea reflects that employees with management authority at cryptocurrency exchanges, no less than the founders of such exchanges, cannot willfully disregard their obligations under the Bank Secrecy Act,” Manhattan U.S. Attorney Damian Williams said.

A spokeswoman for BitMEX said Mr. Dwyer no longer works for the platform.

A lawyer for Mr. Dwyer didn’t immediately respond to a request for comment.

Mr. Dwyer, a 39-year-old citizen of Australia and Bermuda, was among four top BitMEX figures charged in New York. He had pleaded not guilty. Three co-founders, including former BitMEX Chief Executive Arthur Hayes, pleaded guilty earlier this year. Mr. Hayes was sentenced to house arrest, and co-founder Benjamin Delo

was sentenced to 30 months probation.

When prosecutors first filed charges against the four in 2020, BitMEX had grown from a startup to become one of the largest crypto exchanges in the world. Mr. Hayes, who founded BitMEX at age 28, personally earned more than $100 million for his work.

As part of a plea deal with prosecutors, Mr. Dwyer agreed to pay a $150,000 fine. BitMEX itself entered a $100 million settlement with U.S. regulators in 2021.

Write to Richard Vanderford at richard.vanderford@wsj.com