Jang, 67, who became powerful in the country as the uncle of the unpredictable young leader, is the latest victim of what appears to be a power struggle in the isolated Communist nation as Kim, who took over after the death of his father Kim Jong Il in December 2011, consolidates his leadership.
The official Korean Central News Agency said the political bureau of the Central Committee of the Workers' Party of Korea stripped Jang of all posts, depriving him of all titles and expelling him and removing his name from the party.
"Jang and his followers committed criminal acts baffling imagination and they did tremendous harm to our party and revolution," the report said.
KCNA quoted a party resolution that Jang abused his power and challenged the "sole leadership system," gravely hindered the role of the Cabinet by taking control of major economic development sectors, allegedly was guilty of irregularities and corruption, and "led a dissolute and depraved life."
Xinhua, the official news agency of China, which is a close ally of North Korea, said Jang, who was married to Kim's biological aunt, was senior general and vice chairman of the National Defense Commission and secretary of the party's administration department.
The resolution accused Jang of working "to form a faction within the party by creating illusion about him and winning those weak in faith and flatterers to his side."
It said Jang and his followers violated the party's guiding principles and disobeyed "the order issued by the supreme commander of the Korean People's Army."
South Korea's Yonhap News said the KCNA report did not give details on Jang's current whereabouts.
When Jang's removal was first reported last week, South Korea's intelligence regency reportedly told its lawmakers that two of Jang's allies had also been publicly executed.
Yonhap said Jang had long been viewed as Kim Jong Un's guardian. It said Jang's purge could lead to a shakeup of the leadership.
Last year, Ri Yong Ho, chief of the military's general staff, was removed from all his posts because of an unspecified "illness."
A senior South Korean military official earlier told Yonhap Jang's removal could make Kim Jong Un even more powerful but could increase the instability within the regime.
North Korea, under Kim, already has escalated tensions on the Korean Peninsula as it continues to develop its nuclear and missile program despite severe sanctions from the United Nations Security Council. The country conducted its third nuclear in February.
"Some see this as perhaps the last part of the power consolidation phase, that Kim Jong Un has in fact removed all of the old guard close to his father and is now finalizing the inserting of his own inner group," John Park, a Northeast Asia analyst at Harvard University's Kennedy School of Government, told CNN.