A photo of a group of senior Workers' Party officials was published in Thursday's edition of the authoritarian regime's main Rodong Sinmun newspaper.
An article accompanying the front-page photo listed the names of those in the picture. The 20-something Kim Jong Un was one of the officials named and appeared to be sitting near his father with a military officer between them.
The release of the photo comes after the younger Kim earlier this week was handed top military and party posts at a Workers' Party conference.
Lee Jong-joo, a spokeswoman at South Korea's Unification Ministry, which handles relations with North Korea, said a photo of Kim Jong Un had never been published before. She said the young man sitting near Kim Jong Il appears to be Kim Jong Un.
The official Korean Central News Agency announced Tuesday that Kim Jong Un had been promoted to four-star general in the Korean People's Army -- the first mention of his name in the country's tightly controlled state media.
He was mentioned again in several dispatches Wednesday announcing the names of people who were given posts at a rare meeting of the ruling Workers' Party of Korea held the day before. In addition to his military title, he was made a member of the organization's governing central committee and was also named to its military commission.
The mentions of Kim Jong Un in state media -- and now the release of his photo -- have confirmed what many analysts have increasingly believed for about the past two years: that Kim Jong Un has been chosen to succeed his father and carry the ruling family dynasty into a third generation.
Several purported photos of the younger Kim have been circulating for some of that time, mostly obtained and published by Japanese media. Kim Jong Il's former Japanese chef says Kim Jong Un resembles his father in looks, tastes and personality, going so far as to call his the elder Kim's "spitting image."
The unsmiling man sitting near Kim Jong Il in the photo has a round face with his hair apparently parted in the middle and bears a resemblance to his grandfather Kim Il Sung, North Korea's founder.
Kim Jong Il, 68, took over as the leader of North Korea in 1994 when his father Kim Il Sung died of heart failure in what became the first hereditary succession in the communist world.