The yen rose by more than 1 percent against the dollar, and Japan's Ministry of Finance announced that more interventions may follow.
"We are monitoring the situation and are in close contact with the US Treasury Department," said Japanese Finance Minister Satsuki Katayama.
The yen has been weak for a long time and in the spring Japan supported its own currency by buying it, but that has not helped. Support from the US sends a clearer signal.
"It's good for the world economy, but most of all, it's a sign of our friendship," said President Trump.
