In a post on his social media platform Truth Social, Trump said he was pleased that the three countries had signed the joint defence agreement and congratulated their leadership on the development.
He described the pact as a “big, bold and important first step” and said it reflected a growing trend towards greater regional unity and cooperation.
Trump also said the agreement would enable countries in the region to strengthen their ability to defend themselves more effectively.
In a separate statement, the US president said he had instructed the American military to significantly reduce joint military exercises with South Korea, arguing that such drills send an inappropriate and hostile message to North Korea and involve substantial costs for the United States.
Trump referred to his relationship with North Korean leader Kim Jong Un, saying North Korea had not posed a threat to the United States during his presidency and had shown respect.
He also claimed that South Korea had declined to join US efforts aimed at denuclearizing Iran.