The 41-year-old actor also acknowledged that the teen visited him the next month at his Manhattan apartment, where more sexual contact occurred. He told Judge Micki Scherer he knew then that she was 15.
Because the charges are misdemeanors, Barbour will not have to register as a sex offender.
The actor had faced charges of criminal sex act in the third degree, a felony punishable by as many as four years in prison, before the deal.
Before the sentence was imposed, the victim, now 22, told the court that before she reported Barbour about two years ago, she had resigned herself to living with the shame and guilt of the experience.
Because of his actions, she said, "I struggled with complete lack of self-esteem, panic attacks, and put considerable distance between my family and friends for years. I had always blamed myself and felt like I had done something wrong."
She said she was pleased with the sentence and hoped the punishment "will prevent him from even thinking of touching another child again."
Barbour's lawyer, Ronald P. Fischetti, took biting exception to the woman's comments, portraying her as the sexual aggressor and saying her acting career had soared after she filed the complaint against his client.
Fischetti said Barbour and the victim exchanged dozens of friendly e-mails for nearly four years after their last sexual encounter. In many, he said, she thanked Barbour for his mentoring and help in meeting people.
Fischetti insisted that his client was no pedophile, but he conceded that his client, as an adult, should have sent the girl away.
Barbour declined to speak when offered the opportunity. Fischetti's comments caused the girl to weep in court.