SEOUL—An American detained in North Korea said he had spied against the country and asked for forgiveness at a media presentation Friday, nine days after a U.S. tourist was sentenced to 15 years in prison with hard labor for subversion.

Kim Dong Chul told a news conference in Pyongyang that he had collaborated with and spied for South Korean intelligence authorities in a plot to bring down the North’s leadership and tried to spread religious ideas among North Koreans.

Describing his acts as “shameful and ineffaceable,” Mr. Kim said he felt sorry for his crime and appealed to North Korean authorities to show him mercy by forgiving him.

He was born in South Korea and became a naturalized U.S. citizen. In an interview with CNN in January, Mr. Kim said he lived in Fairfax, Va., before moving in 2011 to Yangji, a city near the Chinese-North Korean border. He said he commuted daily to Rason, a special economic zone in North Korea, where he was president of a trade and hotel-services company.

He said in the Pyongyang news conference that he was detained in Rason in October.