Lewin Nyatanga scores in stoppage time as Barnsley condemn Millwall to a second home defeat in four days.
Lewin Nyatanga's 92nd-minute strike has seen Barnsley snatch a 3-2 victory away at Millwall.
The Tykes were nearly ahead on 11 minutes, but David Forde was on hand to keep out a strike from Marley Watkins.
However, it was the Lions, who lost 4-0 against Coventry City on Saturday, who took the lead when Fred Onyedinma fired home from close range after Ed Upson's pinpoint ball teed up the midfielder.
Neil Harris's side could have gone further, but Steve Morison was unable to convert his chance late in the first half.
Instead, it was Barnsley who hit the target as a Watkins cross was met by Sam Winnall, who put his effort beyond the reach of Forde.
Early in the second half, the Tykes were nearly ahead after Shaun Williams was denied by the Republic of Ireland keeper.
Lee Johnson's men managed to find a second on 58 minutes as Alfie Mawson turned home a cross from Marc Roberts.
With Millwall seeking an equaliser in the dying stages, Lee Gregory saw his shot drift agonisingly wide of Adam Davies's goal.
The Lions did get back on level terms when Onyedinma scored his second of the game with a header from Shane Ferguson's cross and looked to have to earn his side a point.
However, Nyatanga scored Barnsley's winner in the second minute of stoppage time, after he responded quickest to Conor Hourihane's free kick before slotting past Forde.
Half-Time Report: Sam Winnall header pegs back Millwall >