Nuclear-armed North Korea fired an 'unidentified projectile' into the sea off its east coast, the South's military said Tuesday.
No further details were immediately available from the South's Joint Chiefs of Staff.
The launch comes just days after North Korean leader Kim Jong Un's influential sister Kim Yo Jong, a key adviser to her brother, dangled the prospect of an inter-Korean summit.
But she insisted that "impartiality" and mutual respect would be required, calling for the South to "stop spouting an impudent remark".
She condemned as "double standards" Southern and US criticism of the North's military developments, while the allies build up their own capacities.
The North was also due to open a session of its rubber-stamp parliament, the Supreme People's Assembly, on Tuesday.
In recent days, South Korean President Moon Jae-in, who has only months left in office, has reiterated his longstanding calls for a declaration of an official end to the Korean War.
The North invaded the South in 1950 and hostilities ceased three years later with an armistice rather than a peace treaty.
Pyongyang is under multiple sets of international sanctions over its banned nuclear weapon and ballistic missile programmes.
It has already carried out several missile tests this month, one involving a long-range cruise missile and another of short-range ballistic missiles, according to the South's military.
Seoul also successfully test-fired a submarine-launched ballistic missile (SLBM) for the first time, making it one of a handful of nations with the advanced technology.
Talks between Pyongyang and Washington have been at a standstill since a 2019 summit in Hanoi between leader Kim and then-president Donald Trump collapsed over sanctions relief and what the North would be willing to give up in return.
The North has subsequently repeatedly excoriated the South and its president Moon, and blown up a liaison office on its side of the border that Seoul had built.