The capture of the boat and its four-man crew on July 30 came while ties between the two Koreas were tense.
North Korea had refused dialogue with the South in anger over Seoul's
conservative leader's move to cut off ties until Pyongyang abandoned its nuclear arms programme.
North Korea, already a pariah state, has become even more isolated by the international community in recent months after a series of missile launches and its second nuclear test which resulted in tighter UN sanctions.
But the release of the boat was the latest in a series of conciliatory moves by the North that also included a deal on Friday to resume reunions of families separated by war and the reopening of its border with the South to commercial traffic.