Carter, 25, entered the plea Tuesday in Orange County Superior Court. The judge also ordered Carter to pay $1,200 in fines, placed him on three years' informal probation and restricted his driving privileges for 90 days, said Deputy District Attorney Tate McCallister.
Carter, who wasn't in court Tuesday, was pulled over March 5 in downtown Huntington Beach. He was charged with one count of driving under the influence and one count of driving with a blood alcohol level above 0.08, the legal limit.
The singer has said through his publicist that he was on prescription medication at the time "and was unaware of its interaction possibilities."
The Backstreet Boys, whose hits include "I Want it That Way," "Shape of My Heart" and "Show Me the Meaning of Being Lonely," recently reunited after four years. Carter, the youngest of the group, released a solo album in 2002.