Barnsley started the game superbly and were ahead in the fourth minute when Jacob Mellis fired a shot home off the underside of the crossbar to shock the visiting Hull supporters.
Barnsley ended the first half the better of the two sides and Hull were fortunate to head into the interval just a goal behind.
However, Hull's bid for automatic promotion was dealt a further blow in the 50th minute when Chris Dagnall combined brilliantly with Chris O'Grady, with the latter extending his side's lead with a deflected strike.
Barnsley's second goal seemed to end Hull's hopes of securing a spot in the Premier League and it was the hosts who continued to look more likely to add to their tally.
The hosts cruised to the three points as the delighted Oakwell crowd celebrated a surprise victory over Steve Bruce's side.