Los Angeles Superior Court Judge Elden Fox issued a stern warning to the 24-year-old actress during the hearing on Friday and ordered her to stay in rehab till January 3.
The 'Mean Girls' star will also be subject to random drug testing until her next court date on February 25, Los Angeles Times reported.
"I'm serious about that (date). You're staying there past the New Year, and there's a reason for that," judge Elden told Lohan.
"I'm not going to be manipulated. If you're serious about what you wrote in the letter, if you're serious about your sobriety... you will admit that you are an addict," Fox added, referring to Lohan's handwritten letter she sent to the judge.
If she completes her treatment and complies with her probation until the February hearing, the actress will be placed on unsupervised probation so she can move on with her life and "put this long episode behind" her.
However, if she violates her probation, she will be sentenced to six months in jail.