NEW YORK—A top U.S. derivatives regulator urged his counterparts at the Securities and Exchange Commission to quickly reach a decision about the status of ether, the world’s second-most-valuable cryptocurrency after bitcoin.

Commissioner Brian Quintenz of the Commodity Futures Trading Commission, which is holding talks on the issue with the SEC, said the agency should quickly provide clarity about whether it plans to label ether as a security—a move that could create uncertainty in cryptocurrency markets.

The...