Deputy Minister of Health Nguyen Truong Son said the person arrived nine days earlier and has been quarantined upon arrival at the 108 Military Hospital.
"The health ministry has informed about the case to the government," Son said.
The infected person landed at Hanoi's Noi Bai Airport on December 19 and tested positive for the coronavirus. They were taken to the 108 Military Hospital soon after.
Genetic sequencing on December 20 and December 21 confirmed that the person is infected with the Omicron, or B.1.1.529 variant, said the ministry.
Omicron, first identified in late November, has raised fresh alarms due to its high number of mutations and higher transmissibility.
Around 80 countries and territories have detected the variant, including many in the Asia-Pacific like Australia, Japan, Singapore and South Korea.