Teenager Reuben Noble-Lazarus recorded his first senior goal to give Barnsley victory over Millwall at Oakwell.
The 17-year-old, who became the club's youngest-ever player in 2008, stepped off the bench to fire the winner with a ferocious 60th-minute left-footed shot.
Nathan Doyle should have added a second but lashed wide having created space.
Millwall went close to an equaliser in the second of six minutes of injury time, Tamika Mkandawire's header from James Henry's cross hitting a post.
The Tykes had to wait until five minutes before the break to test Millwall goalkeeper David Forde for the first time.
He parried away Jason Shackell's glanced header before on-loan Tottenham winger Andros Townsend had a stinging shot saved at the other end.
Barnsley boss Mark Robins opted to introduce Noble-Lazarus for Jacob Mellis at the break - and within 15 minutes the youngster had made a stunning impact.
He collected Doyle's pass and cut inside before drilling a magnificent shot past Forde.