The U.S. Commodity Futures Trading Commission is investigating crypto exchange BitMEX, according to people familiar with the matter, a platform that’s become wildly popular in Asia for letting people make big bets with little money down.

The months-long CFTC probe is focused on whether BitMEX broke rules by allowing Americans to trade on the platform, which isn’t registered with the agency, said one of the people who asked not to be named discussing the investigation, which hasn’t been made public. The regulator considers virtual currencies like Bitcoin to be commodities, and it has jurisdiction over futures and other derivatives based...