The move comes after Kim was formally named head of state and commander-in-chief of the military in a new constitution in July that analysts said was possibly aimed at preparing for a peace treaty with the United States.
North Korea has long called for a peace deal with the United States to normalize relations and end the technical state of war that has existed since the 1950-1953 Korean War, which concluded with an armistice rather than a peace treaty.
Kim's legal status as "representing our state has been further consolidated to firmly ensure the monolithic guidance of
the Supreme Leader over all state affairs," state news agency KCNA quoted Choe Ryong Hae, president of the presidium of the supreme people's assembly, as saying.
The presidium president had historically been the nominal head of state.
But the new constitution said Kim, as chairman of the State Affairs Commission (SAC), a top governing body created in 2016, was the supreme representative of all the Korean people, as well as "commander-in-chief".
A previous constitution simply called Kim the "supreme leader" who commanded the country's "overall military force".
Thursday's constitutional amendments appear to confirm that North Korea's legal system will now recognize Kim as head of state.
The new constitution authorizes Kim to promulgate legislative ordinances and major decrees and decisions and
appoint or recall diplomatic envoys to foreign countries, KCNA said.
"With the amendment, Kim Jong Un is reviving his grandfather's head of state system," said Cheong Seong-chang, a
senior fellow at the Sejong Institute. "He has become a de facto head of state."
In reality Kim, a third-generation hereditary leader, rules North Korea with an iron fist and the title change will mean
little to the way he wields power.
The back-to-back constitutional revision is unprecedented and Kim is emerging as perhaps the most powerful leader since his grandfather Kim Il Sung, who founded North Korea, said Rachel Minyoung Lee, an analyst with NK News, a website that tracks North Korea.
"By further bolstering the SAC chairman's authority, Kim Jong Un is now on par with Kim Il Sung," she said.
Other analysts noted that the moves simply codified the power Kim Jong Un already wields as supreme leader.
"This is more a matter of shuffling the card deck and clarifying a few lines of authority," said Michael Madden, an
expert on North Korean leadership and a fellow at the U.S-based Stimson Center.
"There is no question that Kim Jong Un is the regime's key and - on strategic policy - sole decider," he said.
There has been scant progress in the U.S. aim of getting North Korea to give up its nuclear weapons program despite
three meetings between President Donald Trump and Kim Jong Un.
Trump has said he and Kim agreed at their last meeting to resume working-level talks, although these have yet to happen.
North Korea has since conducted multiple missile tests, while accusing Washington of breaking a pledge to stop joint military exercises with South Korea.