Barnsley dropped out of the Sky Bet League One play-off places after they were beaten by gritty Southend at Oakwell.
Jack Payne’s early strike and a late cracker from sub Michael Timlin proved enough for the Shrimpers, who had an escape when the Reds missed a penalty in the first half.
The Reds largely disappointed in the first half and went in at the break behind.
Payne struck for the Shrimpers after just four minutes. He turned Marc Roberts inside out before guiding a neat shot past Adam Davies from 15 yards.
The visitors’ lead was almost doubled in the 18th minute.
Payne teed up Anthony Wordsworth, but his super curling strike from 20 yards struck the crossbar and bounced away to safety for the Reds.
In the 24th minutes the Reds fluffed a great opportunity to even it up.
Ashley Fletcher was tripped as he tried to round ‘keeper Dan Bentley, but Conor Hourihane disappointingly screwed the resultant penalty-kick wide.
Alfie Mawson saw a well-struck free-kick saved just before the break.
Southend almost went two clear just two minutes after the restart when Kevan Hurst fired into the side-netting from close range.
The Reds soon battled their way back into contention, though, with Fletcher testing Bentley and then Marley Watkins nodding inches wide.
However, sub Timlin smashed in from 25 yards deep in to added time to complete a deserved win for the visitors.
Barnsley (4-4-2): Davies; Connolly, Roberts, Mawson, G Williams; Hammill, Hourihane, Brownhill, Chapman; Fletcher, Watkins. Subs: Townsend, Scowen, Tuton, Nyatanga, Smith, Winnall, Khan.
Southend (4-4-2): Bentley; White, Thompson, Barrett, Coker; Hurst, Deegan, Wordsworth, Kamara; Barnett, Payne. Subs: Smith, Timlin, Atkinson, McLaughlin, McQueen, Loza, O’Neill.
Referee: Darren Handley (Lancashire).