Mark Gower's second half goal put Southend ahead but Coulson headed home after Sam Togwell's initial shot was only parried by Southend keeper Darryl Flahavan.
Barnsley nearly made a dream start on three minutes when Martin Devaney crossed to the unmarked Paul Hayes on the right, but his curling shot hit the right post before he then blasted straight at Flahavan.
The Shrimpers responded with a decent spell that saw Gower's shot deflected out for a corner and Peter Clarke force a decent save out of Tykes goalkeeper Nick Colgan.
Colgan was in action again minutes later when he denied Lee Bradbury after he rose to head Steve Hammell's deep cross from the left.
But Barnsley fought back to produce two good chances of their own. Firstly the unmarked Devaney latched onto a Mark Richards' square ball but flashed his attempt narrowly wide.
Then Togwell released Danny Nardiello who saw his shot from an acute angle fly across the face of goal.
And Nardiello nearly made the breakthrough on the stroke of half-time when his shot was cleared off the line by Clarke and Flahavan pounced on the loose ball before Hayes could react.
Southend took the lead after 58 minutes when Adam Barrett was brought down just outside the box. Freddy Eastwood's low free-kick went through the wall and Gower got a touch that beat Colgan.
Rob Atkinson came close to levelling the score but his close-range header, from Devaney's corner, went agonizingly wide.
And, as both sides battled against a soggy pitch, Gower's point-blank shot was parried by Colgan before Jamal Campbell-Ryce's centre from the right hit the crossbar, sandwiching a long-range shot from Tykes' midfielder Brian Howard that went just over the bar.
But Coulson, who came on 12 minutes from time, ensured Barnsley got a second bite of the cherry on January 16.