Kim Jong-il, North Korea's "Dear Leader", was in the process of grooming Kim Jong-un as his successor when he died on 17 December 2011.
Immediately after his father's death, the younger Kim was hailed as "the great successor". He was named head of the party, state and army within a fortnight of his father's death.
In January 2016, Mr Kim shot to global headlines again when he oversaw what the regime claimed was an underground hydrogen bomb test - its second nuclear test since he came to power.
Previously, his most high-profile act had been to purge and execute his uncle, Chang Song-thaek, who state media said had been plotting a coup, in December 2013.
Little is still known about the elusive young man who is the youngest son of Kim Jong-il and his late third wife Ko Yong-hui.
Born in 1983 or early 1984, he was initially not thought to be in the frame to take up his father's mantle.
Analysts focused their attention on his half-brother Kim Jong-nam and older full brother Kim Jong-chol.
However Kim Jong-nam's deportation from Japan in May 2001 and middle brother Kim Jong-chol's apparent "unmanliness" improved his chances.
Analysts saw him as the coming man after he was awarded a series of high-profile political posts.
Swiss-educated like his brothers, Kim Jong-un avoided Western influences, returning home when not in school and dining out with the North Korean ambassador.
After his return to Pyongyang, he is known to have attended the Kim Il-sung Military University.
His mother was thought to be Kim Jong-il's favourite wife, and she clearly doted on her son, reportedly calling him the "Morning Star King".
In his 2003 book, I Was Kim Jong-il's Chef, a Japanese man writing under the pseudonym Kenji Fujimoto also claimed that Kim Jong-un was his father's favourite.
In August 2010 Kim Jong-il visited China. One South Korean TV station cited a South Korean official as saying Kim Jong-un had accompanied his father on the trip.
Some reports speculated that he had been anointed successor partly because of his resemblance to North Korea's founder Kim Il-sung.
A few North Korea watchers went so far as to say that he may have had plastic surgery to enhance the resemblance, in a country where the deification of the Kim family is at the heart of its grip on power.
Mr Kim made his first public speech as North Korea marked the 100th anniversary of the birthday of Kim Il-sung on 15 April 2012, praising the "military first" doctrine and vowing the time his nation could be threatened was "forever over".
"Superiority in military technology is no longer monopolised by imperialists," he said, adding: "We have to make every effort to reinforce the people's armed forces."
Not much was known of Mr Kim's personal life until television footage of an unidentified woman attending events with him surfaced. In July 2012, state media announced that Mr Kim was married to "Comrade Ri Sol-ju".
Little is know of Ms Ri, but her stylish appearance - short, chic haircut and Western dress - led some analysts to suggest that she was from an upper-class family and that she fits Mr Kim's efforts to project a more relaxed image compared to his predecessors.
Details surrounding the marriage of Mr Kim to Ms Ri remain unclear. Most reports had suggested that Ms Ri may have been a singer who caught Mr Kim's attention during a performance.
Aside from attending official events, the couple's public appearances have included visits to an amusement park and watching a concert featuring Disney characters.
American basketball star Dennis Rodman, who met Mr Kim in 2013 and 2014, told The Guardian newspaper that Mr Kim had a daughter.
In 2012, Mr Kim was appointed marshal - the highest military rank - following a high-level military reshuffle in which army chief Ri Yong-ho was removed.
Since then some of North Korea's actions have elicited condemnation from the international community.
A few months into his leadership, in April 2012, North Korea launched a rocket which it said would put a satellite into orbit. The unsuccessful launch was seen by many as a banned test of long-range missile technology.
This attempt was followed by a successful launch of a satellite into space, using a three-stage rocket, in December 2012. South Korea, Japan and the US said the launch was a disguised missile test, and the UN Security Council unanimously approved a resolution condemning the launch and tightening sanctions.
In February 2013, North Korea carried out a third nuclear test, said to be twice as big as the 2009 test. This resulted in fresh sanctions from the UN Security Council.
In April 2013, heightened tension in the peninsula saw North Korea withdraw workers from the Kaesong industrial zone, jointly run with South Korea and the last symbol of inter-Korean co-operation. The zone was reopened in the following September after negotiations.
In January 2016, the regime made an as-yet unproven claim that it had successfully carried out its first underground hydrogen bomb test, triggering a global outcry and fears that the state, which has few allies, had acquired a more advanced nuclear capability than previously known.
The purge of his uncle in late 2013 caused international concern. Mr Chang was vice-chairman of the powerful National Defence Commission who sat at the heart of the country's leadership, and was seen as a major figure in Mr Kim's administration.
In his first public reference to the execution in a message broadcast on 1 January 2014, Mr Kim spoke of the "elimination of factionalist filth".
Kim Jong-un took on the mantle of North Korea's supreme leadership with little political or military experience behind him.