'Zoey 101' Star Matthew Underwood -- ARRESTED at Hookah Lounge

Former Nickelodeon star Matthew Underwood -- who played Logan Reese on "Zoey 101" -- was arrested outside of his hookah shop in Florida early this morning ... TMZ has learned.

Sources say ... around 3AM, law enforcement in Port St. Lucie rolled up to the Cloud 9 hookah lounge, co-owned by Underwood, and arrested the actor on a probation violation ... stemming from his drug possession conviction earlier this year.

We broke the story ... 22-year-old Underwood had been arrested back in April ... when cops say he was in possession of a ton of weed. Underwood had plead no contest and was sentenced to 12 months probation.

So far, it's unclear why cops believe Underwood violated his probation.

Underwood is currently behind bars at the Port St. Lucie jail -- and is expected to remain in custody ... at least until he sees a judge tomorrow.

10:05 AM PT -- Officials tell TMZ ... Underwood allegedly violated 5 terms of his probation, including:
-- Failed his drug test
-- Failure to complete victim impact panel
-- Failure to complete a substance abuse program
-- Failure to pay court fees
-- Failure to pay monies assessed by the court to his probation officer. He has only paid $5.00 so far.