AP president Tom Curley said the bureau would operate under the same standards as other bureaux worldwide.
All media outlets in North Korea are state-run. Most citizens have no access to the internet or foreign media.
Visits by most foreign journalists are severely restricted and, if granted a visa, reporters are accompanied by government minders to carefully selected locations.
AP said that the bureau would have two permanent North Korean reporters and would be supervised by two Seoul-based US journalists who would make regular visits.
The news agency first established a presence in Pyongyang in 2006, when it opened a video bureau.
The US and North Korea do not have formal diplomatic ties. But the president of state-run KCNA news agency, Kim Pyong-ho, said the two sides had "been able to find a way to understand one another and to cooperate closely enough to open an AP bureau".
The move comes a month after the death of North Korean leader Kim Jong-il. His third son, Kim Jong-un, has been installed as his successor.