On Thursday night, the rate was down to 160 yen per dollar. But suddenly, and without any external explanation, it rose to 155.6 yen per dollar - the biggest gain in almost two years.
The Finance Ministry in Tokyo has not commented, but analysts and media outlets citing sources say the government has intervened to buy yen and sell dollars.
The country's currency chief, Atsushi Mimura, issued a "final warning" to traders selling the yen about the risk of intervention.
The yen has been under heavy pressure from soaring oil prices and hawkish signals from the US Federal Reserve.
A sharp weakening of the yen could force Japan to tighten its monetary policy by raising interest rates.
