WHISTLER, British Columbia—Six finance ministers, representing some of the closest allies of the U.S., issued a stern rebuke of the protectionist policies of the administration of President Donald Trump, signaling a deep rift in the relations underpinning the post-World War II trading system.

The ministers of the six non-U.S. members of the Group of Seven industrialized nations—the host Canada, along with France, Germany, the U.K., Italy and Japan—on Saturday issued a joint statement excluding the U.S., and conveying their...