9:12 AM PT -- The Sitch is on his way to lockup, and just posted a video to all his fans. He promises to be a better version of himself once he gets out and also makes a few plugs for his social media account. Sorrentino will likely be booked within the hour.

Time's up for Mike "The Situation" Sorrentino ... the "Jersey Shore" star is surrendering to begin his 8-month prison sentence.

Sorrentino's attorney, Kristen Santillo, tells TMZ ...he will turn himself in Tuesday to start serving his time at the Federal Correctional Institution in Otisville, New York. We suspect he'll have lots of contact -- written, anyway -- with the outside.

Before reporting to prison, Sorrentino urged friends, family and fans to write him while behind bars ... posting the prison mailing address for his 1.3 million Twitter followers.

As we've reported ... Mike was sentenced to 8 months back in October for tax evasion. Mike's brother, Marc, got 2 years behind bars. The sentencing being pushed to October really helped The Situation.

He was originally supposed to be sentenced in April, but with the extra months of freedom, Mike married Lauren Pesce at a fancy shindig in November. He also got to spend the holidays with friends and fam ... including Thanksgiving.

This Valentine's Day is gonna suck -- no GTL or TLC, we're assuming.

On the upside ... he could run into President Trump's lawyer Michael Cohen, who will also do his time at Otisville, and he'll be out for Halloween.