In a stunning downfall, Jang - who had been seen as Kim's political regent and the country's unofficial number two was executed yesterday immediately after a special military trial, state news agency KCNA reported in a viciously worded attack.
He 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", it said, while also making the rare admission that North Korea is experiencing economic strife.
Jang, 67, played a key role in cementing the leadership of the inexperienced Kim when he succeeded his father Kim Jong-Il in 2011, but analysts said his power and influence had become increasingly resented.
State media showed a stooped and handcuffed Jang, who was married to Kim Jong-Il's sister, being led away from the military trial, flanked by two officers, one of whom had a hand on the back of his neck.
"If confirmed, this is another example of the extreme brutality of the North Korean regime," US State Department deputy spokeswoman Marie Harf said, adding that Washington was following developments closely.
South Korean President Park Geun-Hye has accused Jong-Un of resorting to a "reign of terror" to cement his leadership.
Japan's defence minister warned that the purge could herald upheaval comparable to China's disastrous Cultural Revolution.
The Kim family has ruled the North for six decades with an iron fist, regularly purging those showing the slightest sign of dissent. Most are executed or sent to prison camps, but the nation has not seen such a high-level execution for decades.
The regime accused Jang of betraying the trust of both Jong-Un, who is aged around 30, and his father - saying he had received "deeper trust" from the younger leader in particular.
The KCNA report portrayed Jang as decadent and corrupt, attempting to dominate national affairs by "stretching his tentacles" into ministries and institutions, and responsible for the failings of the hungry and impoverished nation.
"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," it said.