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 large amount of marijuana. Underwood had plead no contest and was sentenced to 12 months 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 today.

Update: 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.