Describing Jang, 67, as a "traitor for all ages," the official Korean Central News Agency said the former senior general was sentenced to death and executed after a special military tribunal found him guilty of treason and committing unforgivable crime as traitor.
Prior to being ousted as part of what appears to be power consolidation by the unpredictable North Korean leader, Jang had been vice chairman of the National Defense Commission and secretary of the Workers' Party of Korea.
KCNA, as quoted by China's Xinhua News Agency, said Jang had been daydreaming about being recognized as a "new regime."
U.S. State Department spokeswoman Marie Harf said in a statement that if confirmed, it would be "another example of the extreme brutality of the North Korean regime." She said the United States is following developments in North Korea and is in consultations with its allies and partners in the region.
Jang's execution follows reportedly similar punishment of at least three more senior officials in the isolated, impoverished Communists country, which, under Kim Jong Un, already has stirred up deep tensions in the Korean Peninsula with its nuclear and missile tests even as it faces tough U.N. sanctions for its provocations and actions.
South Korea's Yonhap quoted North Korea media as saying, "The accused is a traitor to the nation for all ages who perpetrated anti-party, counter-revolutionary factional acts in a bid to overthrow the leadership of our party and state and the socialist system."
KCNA also said Jang committed such a "hideous crime as attempting to overthrow the state by all sorts of intrigues and despicable methods with a wild ambition to grab the supreme power of our party and state."
"Our party will never pardon anyone challenging its leadership and infringing upon the interests of the state and people in violation of the principle of the revolution, regardless of his or her position and merits," the North Korea Workers' Party said.
The party urged all members and the Korean People's Army to be united under Kim's leadership for a "final victory."
Jang's dismissal from all his posts was only officially confirmed on Monday. A party resolution at the time said Jang abused his power and challenged the "sole leadership system," and "led a dissolute and depraved life."
On Thursday, Japan's Mainichi Shimbun said among others recently executed was Ri Su Yong, who had handled the treasury for the late Kim Jong Il, father of the current leader who died in December 2011. However, that could not be confirmed by South Korea's Unification Ministry, South Korea's Chosun Ilbo reported.
There also have been reports that two more of Jang's associates were executed last month after being charged with corruption and disobedience.
At the time of Jang's dismissal, China, North Korea's closest ally, through its Foreign Ministry spokesman, said, "As a friendly neighbor, China hopes (North Korea) to have national stability, economic growth and for its people to enjoy happiness."
Jang was married to Kim Kyong Hui, sister of the later leader Kim Jong Il, and had been seen as the country's second most powerful official and a guardian of the North's current leader, Yonhap said.
"This is a stunning development," Marcus Noland, a senior fellow at the Peterson Institute for International Economics, told CNN. "I've been following North Korea for 20 years and I do not remember them ever publicly announcing the execution of a senior leader. You hear rumors about it, but this theatrical arrest earlier in the week and now this execution are unprecedented."
Noland said it appeared the regime is trying to intimidate anyone who might have independent ideas or be entertaining any ambitions.
John Park at Harvard's Kennedy School of Government told CNN Jang's ouster was perhaps the last part of the power consolidation phase, as Kim Jong Un has removed all of the old guard close to his father and is bringing in his own inner group.
China has been trying to restart the stalled six-nation talks on North Korea's nuclear disarmament. It was not clear how the latest developments would affect those efforts. Other members in the talks include the two Koreas, the United States, Japan and Russia.