The South's military said it could not yet identify the type of missile fired from Pukchang in South Pyongan province but it travelled about 500 km. Washington described it as a medium-range missile.
"Our military is closely monitoring signs for additional provocation by the North Korean military and we are keeping a full military readiness," said a statement from the South's Joint Chiefs of Staff.
The launch came just one week after the North fired a Hwasong-12 intermediate-range missile, which according to Pyongyang flew almost 800 km and could carry a "heavy" nuclear warhead. Analysts said the Hwasong travelled further than any previous ballistic missile launched by the North.
The May 14 launch was seen as a significant step forward in the North's weapons capabilities as it accelerates efforts to develop an intercontinental ballistic missile (ICBM) capable of delivering a nuclear warhead to the continental United States.
The launches, and a threatened sixth nuclear test, have fuelled tension with the administration of US President Donald Trump, who has vowed that such an ICBM launch "won't happen".
South Korea's new President Moon Jae-In called a National Security Council meeting in response to the latest launch, Yonhap news agency reported.
A White House official visiting Saudi Arabia with Trump confirmed the North had test-fired a medium-range ballistic missile but appeared to play down Sunday's launch.
"We are aware that North Korea launched an MRBM. This system, last tested in February, has a shorter range than the missiles launched in North Korea's three most recent tests," the official said on condition of anonymity.
Pyongyang has long had missiles that can reach targets across South Korea and Japan.
With an imputed range of 4500 kilometres the Hwasong-12 also puts US bases on the Pacific island of Guam within reach.
