Lindsay Lohan was released from a county jail in Lynwood, Los Angeles, in the early hours of Monday morning.
The actress served a total of 13 days for violating probation on a 2007 conviction for driving under the influence.
A sheriff told waiting media that the star would go directly to a substance abuse treatment centre, but did not reveal which one: "The court order specified that certain named people would pick her up. It's our understanding she went directly to a treatment centre."
Twenty-four-year-old Lohan had her jail term reduced for good behavior and because of a program to relieve jail crowding.
She was originally given a 90 day sentence ordered by Beverly Hills Judge Marsha Revel.