Despite making two appearances for Newcastle at the start of the 2016–17 season, on 30 August, Armstrong joined fellow Championship side Barnsley on an initial six-month loan deal, having signed a new four-year deal at St James' Park before his departure, after Newcastle manager Rafael Benítez expressed his desire for Armstrong to gain more experience at Championship level. He made his debut for the Tykes on 10 September as a substitute in place of Ryan Kent during a 2–1 victory at Preston North End, scoring the winning goal with a left-footed curling effort after outpacing and losing his marker with a backheel. In January 2017, it was announced that Armstrong had extended his loan spell at the club until the end of the 2016–17 season.
