Lindsay Lohan checked into the Century Regional Detention Facility in Lynwood, CA at 8:48PM PST to begin serving her 30 day jail sentence ... according to the L.A. County Sheriff's Department website.
As TMZ previously reported, Lindsay is expected to be released within hours of her arrival due to jail overcrowding.
Sheriff's spokesman Steve Whitmore has said LiLo might have to spend overnight behind bars ... IF there are several other inmates checking out at the same time. LA traffic.
For now ... she's still in custody.