Ayoze Perez, Chancel Mbemba and Dwight Gayle scored in a comfortable victory, but it was Jack Grealish’s 89th-minute goal which sealed the Magpies’ title.
Perez’s flicked finish made it 1-0, before Mbemba smashed in to double the lead from a Perez rebound.
Gayle added a late third before Grealish’s goal handed them the title.
It was a comfortable win for Rafael Benitez’s side – yet it looked like it would not be enough to seal top spot, after Glenn Murray’s penalty had put league leaders Brighton in front at Villa Park.
However, despite being down to 10 men following Nathan Baker’s red card, Grealish beat Brighton keeper David Stockdale to send the Magpies above Brighton in the table.
At St James’ Park, DeAndre Yedlin terrorised Barnsley down the right-hand side and it was his cross which Perez guided into the bottom corner to open the scoring.
Christian Atsu forced Barnsley goalkeeper Adam Davies into two strong diving saves with powerful shots and Massadio Haidara smashed a good chance over the crossbar from Perez’s cut-back as the hosts dominated.
Davies kept out Jack Colback with his legs, but the Magpies got a deserved second when Davies pushed Perez’s shot out perfectly for Mbemba to smash home his first goal for the club.
Jonjo Shelvey struck the post before Aleksandar Mitrovic’s header sent substitute Gayle through to complete the scoring with a confident finish past the onrushing Davies.
Mission accomplished for Newcastle
Under manager Benitez, Newcastle brought 12 players to the club after dropping into the Championship last summer, with Matt Ritchie and Gayle moving down a division to sign five-year deals.
More than £50 million was spent in transfer fees alone, but these were offset by the sales of Andros Townsend, Moussa Sissoko and Georginio Wijnaldum among others.
Nevertheless, the Magpies were still under pressure to achieve promotion – a feat they managed with two games to spare.
Benitez’s side have battled with Brighton throughout the season for top spot, but since the Seagulls achieved promotion on 17 April they have dropped off the pace dramatically.
Ultimately it was three straight wins for Newcastle and three matches without three points for Chris Hughton’s side that told, giving the Toon the perfect end to the campaign.
