The US Army identified the soldier as Private Travis T. King who joined in 2021 and was facing disciplinary action.
While on an orientation tour of the Joint Security Area (JSA) on the border between the two Koreas, King crossed into North Korea on Tuesday "wilfully and without authorization," US Defence Secretary Lloyd Austin said.
"We believe that he is in (North Korean) custody and so we're closely monitoring and investigating the situation and working to notify the soldier's next of kin," Austin told a briefing, Reuters reported.
North Korea's state media has made no mention of the incident. Its mission to the United Nations in New York did not immediately respond to requests for comment.
The crossing comes at a time of renewed tension on the Korean peninsula, with the arrival on Tuesday of a US nuclear-armed ballistic missile submarine, and the test launch early on Wednesday of two ballistic missiles into the sea by North Korea.
The UN Command (UNC), which oversees security for the border area, had communicated over a hotline with the North Koreans about the U.S. soldier, a spokesperson for US Forces Korea said.
The US military was "working with our KPA counterparts to resolve this incident," Colonel Isaac Taylor said, referring to North Korea's People's Army.
"We communicate with the North Koreans every single day," he said. "It's all part of the armistice agreement."
MNA/PR