The reactor has been offline since 2007 as part of international nuclear disarmament talks that have since stalled.
A spokesman for the North's General Department of Atomic Energy said scientists will quickly begin "readjusting and restarting" the facilities at its main Nyongbyon nuclear complex, including the plutonium reactor and a uranium enrichment plant. Both could produce fuel for nuclear weapons.
North Korea said work to restart the facilities would begin "without delay." Experts estimate it could take anywhere from three months to a year to reactivate the reactor.
In recent weeks, North Korea has threatened to launch attacks against the U.S., declared a state of war on the South and pronounced nuclear weapons a top priority.
The latest announcement will boost concerns in Washington and among its allies about North Korea's timetable for building a nuclear-tipped missile that can reach the United States, although it is still believed to be years away from developing that technology.
On Monday, the Pentagon moved a guided missile destroyer and high-tech radar device into the waters off the North Korean coast.
Officials believe young North Korean leader Kim Jong Un is trying to intimidate his neighbors and bolster his image as a powerful military commander at home.
U.N. Secretary-General Ban Ki-moon said North Korea appears to be "on a collision course with the international community." Speaking in Andorra, the former South Korean foreign minister said the crisis has gone too far, and international negotiations are urgently needed.
Even China, North Korea's only major economic and diplomatic supporter, expressed disappointment.
"We noticed North Korea's statement, which we think is regrettable," Chinese Foreign Ministry spokesman Hong Lei said.
The Associated Press contributed to this report.
