Full-time score from Oakwell
Millwall's 2013/14 away day woes continued at Oakwell as Jim O'Brien's early goal handed Barnsley a 1-0 win over The Lions.
Midfielder O'Brien slotted home in the fourth minute and Millwall, despite dominant periods, couldn't find a way past Barnsley keeper Luke Steele on a day when the Championship's bottom three all won.
Ian Holloway made three changes from the side who drew 1-1 with Bolton last weekend, bringing Shane Lowry back into defence after suspension and handing Scott McDonald and DJ Campbell starts as part of a front three.
But the game had barely settled into any kind of rhythm when the hosts took the lead in the fourth minute. A long ball forward was controlled expertly by O'Brien in the box, and he edged past David Forde before slotting the ball through the legs of Scott Malone who had retreated to the goal line.
That settled Danny Wilson's side's nerves, and they should have made it 2-0 soon after, only for Dale Jennings, in behind the defence, to fire straight into the grateful arms of Forde moments after Chris O'Grady had skied a good opportunity.
Having ridden an early storm, however, The Lions began to control proceedings with Ed Upson and Shaun Williams dictating the tempo in the centre of midfield. They were both involved in the build-up to Millwall's first real chance, but Barnsley managed to clear following some pinball in the box.
Martyn Woolford, increasingly involved in an advanced midfield role, brilliantly dribbled past two Tykes markers on the edge of the box, but his through ball to Campbell in the 20th minute was frustratingly overhit.
Campbell then miscued a good chance with Steele off his line, before Upson fired over the bar from 20-yards.
It was all Millwall as the half wore on, but The Tykes did enjoy a brief spell of pressure on Forde's goal before the interval; Shane Lowry flicking Peter Ramage's header away from danger after Jennings' corner.
Holloway threw on Liam Feeney for Upson early in the second-half, but it was the home side who were now on top, with substitute Stephen Dawson twice firing over from distance.
McDonald and O'Grady were then guilty of spurning good opportunities at either end; the strikers failing to connect cleanly with crosses into the box.
The Lions pushed forward, and Williams and Campbell both came close to levelling, but Barnsley stood firm and an all-important equaliser looked increasingly unlikely. Williams did come agonisingly close with a free-kick though, delicately curling the ball just wide with Steele sprawling across his line.
Lions boss Holloway brought on Fede Bessone for his debut in a last ditch attempt to change the game, but Barnsley held firm to secure a vital three points, and with Charlton and Yeovil both also winning, Millwall slipped to within a point of the dreaded drop zone.
Barnsley: Steele, Ramage, Kennedy, Cranie, O'Brien, Mellis (Dawson, 46'), Hunt, Frimpong, Jennings, Proschwitz (Shea, 63'), O'Grady. Subs not used: Turner, Woods, Lawrence, McCourt, Cywka
Goalscorers: O'Brien 4'
Bookings: Frimpong 86'
Millwall: Forde, Shittu, Morison, Woolford, Lowry, Fredericks, Upson (Feeney, 57'), Williams, Campbell, McDonald (Jackson, 73'), Malone (Bessone, 85'). Subs not used: Bywater, Dunne, Easter, Abdou
Bookings: McDonald 69', Bessone 89'
Referee: Mark Brown