The Hyundai engineer, who has been held since March, was released after the company's chairwoman traveled to Pyongyang Monday.
US envoy Stephen Bosworth is visiting Beijing, Seoul, and Tokyo, after Pyongyang rejected six-party dialogue and threatened to test another nuclear bomb.
The missile launch is believed to be a military test, though Pyongyang insists it is launching a satellite.
The move comes as South Korea and the US begin a large-scale joint military exercise.
The Monitor's language columnist tries to do her bit for economic literacy.
Missile launches and the expulsion of South Korean personnel from a joint industrial zone show Pyongyang's irritation with Seoul.