North Korea fired a ballistic missile into waters from its west region, South Korea's Joint Chiefs of Staff (JCS) said.
Projectile was launched from South Hwanghae province, Yonhap news agency quoted the JCS as saying.
More r details are being analysed, the official said.
Launch came after North Korea test-fired two medium-range Rodong ballistic missiles and one shorter-range Scud missile on July 19.
Japan has lodged an official protest with Pyongyang against the missile launch.
It said that the launch was in defiance of UN Security Council resolutions.
Defense Minister Gen Nakatani said the missile landed within Japan's exclusive economic zone in the Sea of Japan.
He added that his agency was currently "analysing the situation".