Following a "bitter lesson", Hilton plans to do charity work and discard the 'dumb blonde' image, according to reports.
The original 45-day term set by the judge was effectively cut in half under a standard credit applied for good behaviour.
"This is by far the hardest thing I have ever done," she said in a statement. "I have had a lot of time to reflect, and have already learned a bitter but important lesson from this experience."
Hilton has previously revealed that she has been changed by her prison experience, and that God has given her "this new chance" to make a difference.
"I used to act dumb. ... That act is no longer cute," ABC quoted Hilton as saying to American media personality Barbara Walters.
She has also released a statement saying she hoped the media would focus on "more important things" than her 45-day jail sentence.
She added: "Now, I would like to make a difference. ... God has given me this new chance."
After spending three days in jail in a reckless driving case from 3 June, Hilton was briefly released to home confinement for an undisclosed medical condition. She was then sent back to jail little over a day later by an outraged judge, and was housed in the medical ward of a maximum-security detention centre. She was later transferred to an all-women detention centre.
