"He also knows that I am with him and does not want to break his promise to me," President Trump wrote in the social media post.
Firing a short range missile would not violate North Korea's promise not to test long range or nuclear missiles.
But Pyongyang appears to be growing impatient with Washington's insistence that full economic sanctions remain until Mr Kim takes serious steps to dismantle his nuclear weapons programme, says the BBC's Laura Bicker.
"We are aware of North Korea's actions tonight," said White House spokeswoman Sarah Sanders. "We will continue to monitor as necessary."
Nuclear test pledge
North Korea "fired a number of short-range missiles from its Hodo peninsula near the east coast town of Wonsan to the north-eastern direction from 09:06 (00:06 GMT) to 09:27," the South's Joint Chiefs of Staff said in a statement.
The missiles flew for between 70km and 200km (45-125 miles) before landing in the Sea of Japan, they added.
Hodo has been used in the past for launching cruise missiles and long-range artillery testing.
According to the North Korea news agency (KCNA), April's test of a new "tactical guided weapon" was overseen by Mr Kim himself. It said the test was "conducted in various modes of firing at different targets", which analysts believe means the weapon could be launched from land, sea or air.
It is unclear if that weapon was a missile, but most observers agree that it was probably a short-range weapon.
Last year, Mr Kim said he would stop nuclear testing and would no longer launch intercontinental ballistic missiles.
Nuclear activity appears to be continuing, however, and satellite images of North Korea's main nuclear site last month showed movement, suggesting the country could be reprocessing radioactive material into bomb fuel.
The country claims it has developed a nuclear bomb small enough to fit on a long-range missile, as well as ballistic missiles that could potentially reach the mainland US.