A U.S. appeals court said Venezuela’s stake in U.S.-based oil refiner Citgo Petroleum Corp. could be seized to satisfy a judgment against the country, dealing a blow to its U.S.-backed opposition government.

The U.S. Court of Appeals for the Third Circuit on Monday sided with Crystallex International Corp., a defunct Canadian gold miner that has laid claim to Citgo’s valuable Gulf Coast crude refineries to collect on a $1.4 billion debt.

The...