The US Pacific Command (PACOM) said the missile was fired near Wonsan, on North Korea's eastern coast.
"The missile was tracked for six minutes until it landed in the Sea of Japan," it said in a statement.
"We are working with our interagency partners on a more detailed assessment. We continue to monitor North Korea's actions closely."
The statement affirmed the American commitment to stand behind Japan and South Korea, and said the missile was never a threat to North America.