The report added that Park had expressed "sincere repentance" for his actions. However, it did not specify where or when he would be freed.
"According to the results of the investigation, he trespassed on the border due to his wrong understanding of the DPRK [North Korea]," the news agency said.
"The relevant organ of the DPRK decided to leniently forgive and release him, taking his admission and sincere repentance of his wrongdoings into consideration."
"I would not have committed such crime if I had known that the DPRK respects the rights of all the people and guarantees their freedom and they enjoy a happy and stable life"
quoted by North Korea's KNCA news agency
North Korea announced on December 29 last year that it had detained an unnamed US citizen, but said nothing more until Friday, the first time it confirmed that it had been holding Park.
KCNA reported late last month that authorities had detained a second US citizen, also for illegally entering the country from China.
The man, who has not been named, was said to be under investigation.
US officials have said they are seeking consular access to the man, although the US has no formal diplomatic ties with North Korea.
Park's release comes amid recent statements by the North of its intent to improve relations with the US after decades of hostility.
Last year two other Americans, journalists Laura Ling and Euna Lee, were freed after earlier being arrested at the border and sentenced to 12 years hard labour.
Meanwhile, North Korea's relations with its neighbour South Korea have worsened after North Korean forces fired artillery near their disputed western sea border.
Late last month, the North fired about 30 artillery rounds into the sea from its western coast, triggering a brief exchange of fire with a nearby South Korean marine base.
Announcing that Park would be released KCNA also carried a lengthy interview with the US missionary, who was quoted as saying he was taken in by the West's "false propaganda" about rights abuses and a lack of religious freedom in the North.
|North Korea said last month it had detained a second unnamed US citizen [Reuters]
"I trespassed on the border due to my wrong understanding of the DPRK caused by the false propaganda made by the West to tarnish its image," he was quoted as saying.
The DPRK is the acroynm for North Korea's formal name, the Democratic Republic of North Korea.
"I would not have committed such crime if I had known that the DPRK respects the rights of all the people and guarantees their freedom and they enjoy a happy and stable life."
Park was also quoted as saying that he had been treated "in a kind and gentlemanly manner" and that religious freedom was "fully ensured" in the North.
According to the KCNA report, Park said he had been allowed to pray daily, had had his Bible returned to him by authorities and had been allowed to attend a service at Pyongyang's Pongsu church.
US and United Nations officials, along with international rights groups, have strongly criticised the North's human rights record.
Jo Sung-rae, a South Korean Christian rights activist involved in Park's case, said he did not trust the KCNA interview.
"When he comes out and is interviewed again, what he truly thinks will come out from the bottom of his heart," Jo told the AFP news agency.