Pro Bowl running back Dalvin Cook has officially begun his contract holdout with the Vikings ... and, according to ESPN's Adam Schefter, he ain't stopping it until he gets a new, "reasonable" deal.

Cook played out the first 3 seasons of his rookie contract in Minnesota with huge success ... piling up more than 2,000 rushing yards despite injuries limiting him to just 29 games.

But, he clearly isn't willing to go back out for Minnesota for the final year of his rookie deal ... with Schefter reporting the RB is now done with all team workouts until he puts pen to new paper.

The move is pretty surprising ... remember, TMZ Sports just spoke with Dalvin in March out in NYC, and he sure didn't seem like a guy gearing up for a nasty contract negotiation.

Then again, the 24-year-old IS severely underpaid ...

Cook is set to make just $2 million this season ... whereas other top RBs like Christian McCaffrey, Ezekiel Elliott and Le'Veon Bell are slated to make over $13 mil.

Unclear what numbers Dalvin is looking for in the new contract ... but it's safe to say $13 million is probably the STARTING point.

If the Vikings ain't able to meet those demands ... is it officially Alexander Mattison season in Minny?