It came just hours after South Korean President Moon Jae-in attended a ceremony for a railway line he hopes will eventually connect the divided Korean peninsula.
It was North Korea’s first such launch since October and comes amid a stall in talks between Pyongyang and Seoul.
"The United States condemns the DPRK’s ballistic missile launch," a State Department spokesperson said in a statement that also called for rewewed dialogue with Pyongyang.
UN Security Council resolutions ban all ballistic missile and nuclear tests by North Korea, and have imposed sanctions over its development programmes.
Military officials fired the missile at around 8.10am from an inland location, over the coast and into the sea, South Korea’s Joint Chiefs of Staff (JCS) said.
It came just hours after President Moon visited the east coast city of Goseong, which lies near the border with the north. During his visit, he hailed a new railway line opening in the town as a “stepping stone for peace and regional balance” on the peninsula.
"We should not give up the hope for dialogue in order to fundamentally overcome this situation," he said.
"If both Koreas work together and build trust, peace would be achieved one day."
Ankit Panda, of the Nuclear Policy Programme at the Carnegie Endowment for International Peace, told BBC News that there was “no way to assess” whether it could have been a longer-range missile flown on a shortened trajectory.
Japan’s defence minister said the suspected ballistic missile had flown an estimated 310 miles.
"Since last year, North Korea has repeatedly launched missiles, which is very regrettable," Japanese Prime Minister Fumio Kishida told reporters.
It comes after Kim Jong-Un said the north would continue to strengthen its military at an end-of-year meeting of his party.
South Korean officials have claimed that North Korean troops have been conducting winter exercises for the past several weeks.
"Our military is maintaining readiness posture in preparation for a possible additional launch while closely monitoring the situation in close cooperation with the United States," the South’s JCS said in a statement.