Aijalon Mahli Gomes, from Boston, attempted to commit suicide and was being treated at a hospital, the North's official Korean Central News Agency reported, citing information from "a relevant" government entity.
KCNA said the attempt was "driven by his strong guilty conscience" and despair that the U.S. government has not tried to gain his freedom.
The brief dispatch also said the Swedish embassy in Pyongyang was aware of his condition. The U.S. and North Korea do not have diplomatic relations, and Sweden handles Washington's interests in the country.
In Stockholm, the Swedish Foreign Ministry declined to comment, referring queries to the U.S. State Department.
Gomes was sentenced in April to eight years of hard labor and fined $700,000 for entering the country illegally and for an unspecified "hostile act."
The reported suicide attempt came weeks after North Korea threatened to increase punishment for Gomes amid tensions over the deadly sinking of a South Korean warship that Seoul and Washington have blamed on Pyongyang.
KCNA reported in June that the North was examining what harsher measures to take against Gomes under a wartime law, and would be compelled to consider applying the law if the U.S. persisted in its "hostile approach."
North Korea has freed three other Americans detained for illegal entry, but ruled out Gomes' release amid the ongoing tensions.
Diplomats say the U.N. Security Council is set Friday morning in New York to approve a statement condemning a deadly torpedo attack on the South Korean warship that killed 46 sailors, but the declaration stops short of directly blaming North Korea.
The North denies it was responsible and has warned any punishment would trigger war.
Gomes's motivation for entering North Korea is unclear. He had been teaching English in South Korea before being arrested in the North on Jan. 25.
In late April, he was allowed to speak to his mother by telephone.
Gomes had attended rallies in Seoul in support of Robert Park, a fellow Christian who crossed into North Korea from China to call attention to the North's dismal human rights record.
Park was expelled from North Korea about 40 days after entering the country last Christmas.