Baggies boss Tony Mowbray saw his side concede two first-half goals and stormed off to the dressing room even before the half-time whistle.
But after Mowbray's blast at the break the promotion favourites produced a second-half fightback and came close to salvaging a point.
Goals from Danny Nardiello and Jon Macken put Barnsley in the driving seat but James Morrison hit back for the visitors.
The first-half advantage was no more than Barnsley deserved after they bounced back from a 4-0 thrashing by Coventry a week earlier.
The home side could have shocked the Baggies as early as the first minute when Anderson Silva's ball into the box came off Macken and fell to strike partner Nardiello who snapped in a shot from 12 yards that beat Dean Kiely only for the ball to hit the foot of the post.
And as Simon Davey's side rattled West Brom the home side went close with long-range shots from Diego Leon, Brian Howard and Silva.
Nardiello cashed in as West Brom dithered when Bostjan Cesar dawdled after Paul Robinson's casual back-pass.
The quickfire striker - on loan from QPR - nipped in and raced into the area before clipping a cool shot past Kiely for his first goal since returning to the club he left last summer.
And worse was to come for West Brom, backed by a noisy 4,000 following. It came in first-half stoppage time when West Brom panicked in the area after Silva's shot.
Martin Devaney hit a follow-up shot that struck Jonathan Greening on the line and then Macken drove in the close-range shot to double the lead.
West Brom could only muster rare attempts but a header from Martin Albrechtsen almost crept through a crowd of players after a Chris Brunt corner.
Mowbray's half-time blast at his players was followed by a double half-time substitution and it fired up the visitors after the break.
Eight minutes after the restart Morrison conjured up his side's first attempt on target - a shot that brought a full-stretch save from Tony Warner.
Then Warner produced a brave point-blank stop, hurling himself to block a fierce shot from sub Leon Barnett.
Barnsley's resistance finally cracked after 58 minutes when West Brom produced their slickest move of the afternoon.
Morrison's quick pass to top scorer Kevin Phillips was back-heeled to Morrison who rifled in a low shot to pull one back and trigger a grandstand finish.