Nineteen-year-old Armstrong will link up with Paul Heckingbottom’s side on his return from international duty with England Under-20s, who he is currently with ahead of the Young Lions’ friendly against Brazil on Sunday.
The Magpies striker could make his debut for the Tykes when they face Preston at Deepdale in a Sky Bet Championship clash on Saturday, 10th September, and will stay at Oakwell until Tuesday, 3rd January 2017.
Under the terms of his loan, Armstrong will be ineligible to play when Newcastle face Barnsley at Oakwell on Tuesday, 18th October.
Armstrong spent last season on loan at Coventry City in League One, scooping the Sky Blues’ top scorer award thanks to his 20 goals in League One – the joint fifth highest total in the third tier.
Last week, the teenager signed a new deal with the Magpies that will keep him at St. James’ Park until 2020.
He has made two substitute appearances for Rafa Benitez’s side so far this season, coming off the bench against Fulham and Huddersfield Town.