Activists from South Korea said Park entered North Korea from China to call attention to the country's human rights conditions.
They said he walked across the frozen Tumen River from China.
Jo Sung-rae of the Seoul-based activist group Pax Koreana said that North Korean border guards detained the missionary as soon as he walked into the communist state.
'Vision from God'
According to his colleagues, Park, a US citizen of Korean ancestry, claimed he had seen a vision from God of North Korea's liberation and redemption.
|Euna Lee and Laura Ling were freed by the North after Bill Clinton intervened [Reuters]
They also said he crossed the border shouting "I came here to proclaim God's love".
Park is said to have carried a letter calling on Kim Jong-il, the North Korean leader, to release political prisoners, close concentration camps and take steps to improve rights and conditions.
His crossing comes after North Korea freed two US journalists earlier this year who had been arrested in March and sentenced to 12 years of hard labour for trespassing and "hostile acts".
They were freed in August following a diplomatic mission led by Bill Clinton, the former US president.
North Korea holds some 154,000 political prisoners in six large camps across the country, according to South Korean government estimates.
The country has long been regarded as having one of the world's worst human rights records, but it denies the existence of prison camps.