8:55 AM PT -- Weiner is officially in custody. We're told he was dropped off Monday morning at the prison in northern Massachusetts.
Anthony Weiner will begin serving his prison sentence Monday for sexting with a 15-year-old girl.
Weiner was sentenced to 21 months for the crime of transferring obscene material to a minor. He'll serve his time at the Federal Medical Center in Devens, Massachusetts. The facility offers treatment for sex offenders.
Weiner could get 15% of the 21 months shaved off for good behavior.