Prosecutors say 21-year-old Norman Stan Chee entered his plea Tuesday in federal court in Las Cruces and didn't have a plea agreement.
Chee was arrested last October based on a criminal complaint alleging that he knowingly engaged in a sexual act with a minor on the Mescalero Apache Reservation.
Authorities say Chee's sexual relationship with the victim began in February 2012 and he admitted knowing that the victim was 15 years old.
Chee is in federal custody and remains detained pending his sentencing hearing, which has yet to be scheduled.
Prosecutors say he faces a maximum sentence of 15 years in federal prison.