Tom Bradshaw made it seven goals for the season with a double at Millwall
Tom Bradshaw's double helped Barnsley to their first away win of the season at Millwall.
Bradshaw tucked in Harvey Barnes' cross from close range before Lee Gregory levelled with a penalty after the striker was felled by Matty Pearson.
Barnsley boss Paul Heckingbottom was sent off just after half-time, but Bradshaw tapped in his second, linking again with Barnes.
And Mamadou Thiam's penalty sealed the win after Jared Bird was brought down.
Gregory - making his 150th appearance for the Lions - had a good early chance for Millwall, but despite coming into the match with just one point and three goals on the road this season, the Tykes held their own in a first half low on quality.
Heckingbottom was sent to the stands after referee Oliver Langford was summoned to the touchline by the fourth official, but it had no negative effect on the visitors.
Steve Morison's header forced a point-blank save from Adam Davies, but the visitors had chances to further their lead as Millwall continued to chase an equaliser, with Brad Potts' header denied by a brilliant Jordan Archer save.
The victory may have bucked the form book this season, but it extended Barnsley's run of victories over the Lions to five, including the 2015-16 League One Play-off final.
The three points lifted Barnsley up three places to 18th, while Millwall drop to 13th after their first defeat in five Championship matches.
Barnsley head coach Paul Heckingbottom: "I made sure none of the players went up to the referee at half-time. I just approached him by myself.
"When I came out in the second half to go in the technical area he told me I couldn't go in there. I didn't want to make a scene, but I don't know why I wasn't allowed to be in there in the second half.
"It was a clean tackle (from Matty Pearson), we could all see it. When I watched it back it is so apparent with the Millwall players' reactions, but the linesman's flag went straight up."
Millwall manager Neil Harris: "I was disappointed with the performance of course, and disappointed with the outcome result-wise. I take responsibility because I picked the team.
"If I were to take anything away from today, it's if we're not at our best Millwall standard in every fixture in the league this season, we can get beat any time - home or away."
