Jang, believed to be in his 50's, replaced Gen. Kim Kyok Sik, 75, as minister of the People's Armed Forces, South Korea's national news agency Yonhap said.
The ministry is controlled by Pyongyang's powerful National Defense Commission, the Yonhap report said.
The appointment was made during an official function of the Korean People's Internal Security Force attended by North Korean leader Kim Jong Un and his wife, said Yonhap, which monitored a report by North Korea's tightly government-controlled news agency Korean Central News Agency.
The Yonhap report said Kim is noted for being a hard-liner and was involved in the shelling of the South Korean island Yeonpyeong in the Yellow Sea in November 2010.
South Korea said the unprovoked attack killed two marines and two civilians and damaged houses and military buildings.
Kim also was allegedly involved in the sinking of the South Korean naval vessel Cheonan eight months earlier when he was in charge of the army in Hwanghae province on the west coast, the Yonhap report said.
The 1,200-ton naval corvette Cheonan sank rapidly after an explosion from a suspected torpedo ripped the vessel in half in March 2010. It sank just more than 1 mile southwest of Baeknyeong Island near the de facto sea border with North Korea.
North Korea consistently denies it had anything to do with the sinking.
Jang -- promoted to major general in April 2002 and lieutenant general in November 2011 -- was reportedly commander of the army in Kangwon province on the Korean Peninsula's east coast.
The Yonhap report quoted an unnamed "expert" in Seoul saying Jang "was considered a member of the junior faction within the military and his sudden emergence may be a sign that the North Korean leadership is seeking a change to younger officers."
He said Kim's replacement means most senior North Korean officers in their 70's have been retired.
Another analyst, Chang Yong-seok, senior researcher at the Institute for Peace and Unification Studies at Seoul National University, said the appointment "could be seen as an attempt to get younger people to exercise more control over military matters."
The announcement comes as the nuclear-powered U.S. aircraft carrier USS Nimitz left a South Korean port Monday to take part in naval exercises with the South Korean navy in the Sea of Japan.
South Korea deployed its King Sejong destroyer, a warship equipped with an Aegis ballistic missile defense system, to take part in the exercises.
North Korea condemned the exercises as "a grave military provocation to unleash a nuclear war."
The Nimitz had spent a weekend on a port call in Busan, South Korea, a report by The Stars and Stripes said.
"The operations include integrated flight operations, air defense events, surface warfare training events, precision ship maneuvers and liaison officer exchange," a U.S. Navy 7th Fleet statement said.
South Korean President Park Geun-hye met last week with U.S. President Barack Obama to discuss the security situation in the region.