Yonhap News Agency quoted an unidentified South Korean government official as saying that the drone was two to three metres long and comprised a Japanese engine and Chinese parts, as well as a small camera.
The news agency said the drone was similar to one found in a city near South Korea’s border with North Korea last month.
North Korea released TV footage last year of practice drones that had been modified to crash into pre-determined targets but it is not believed to operate drones capable of air strikes or long-range surveillance flights.
However, last month North Korean state news agency KCNA revealed pictures which it said was North Korean leader Kimg Jong-un at a military drills of drones assaulting targets (see picture, above).
Sabre-rattling between North and South Korea has intensified in recent weeks, prompting China to urge restraint.
On Monday, North Korea fired 100 artillery shells into South Korean waters, prompting the South to respond with 300 shells.
North Korea has said it is being provoked by a joint US/South Korea military drill, and by UN condemnation of North Korean missile tests.
A military official told Reuters that South Korea was now investigating where the drone had come from and what its purpose might have been.