The Reds and Lions play out a stalemate in Oakwell's first fixture since the return to play.
Reds' Head Coach, Gerhard Struber remained faithful to his starting lineup from last weekend who'd beaten QPR on their own patch, but couldn't find the target on this occasion.
In a game of few chances, the first half in particular was scrappy with little entertainment on offer.
A Cauley Woodrow free-kick from the edge of the box was viciously heading towards goal before being turned over by Lions 'keeper Bartosz Bialkowski.
The away side's best effort was also via a free-kick from the edge of the opposite box, as Shane Ferguson saw a superb effort crash against the underside of the crossbar, hitting Jack Walton on the way back out, who smothered any potential rebound.
There was a late strike before the break from Marcel Ritzmaier but it flew a foot over the bar and saw us head into the interval goalless.
HALF TIME: Barnsley 0-0 Millwall
As was the case last week, Struber brought on Conor Chaplin for Elliot Simoes during the half-time break.
Barnsley started brightly, with Alex Mowatt forcing Bialkowski into a diving save to his right from the skipper's 20-yard effort.
Both Michael Sollbauer and Jacob Brown had tame efforts when well-placed as the game went beyond the hour-mark, before Millwall captain Jake Cooper slid in to deny Chaplin inside the box a clear scoring opportunity.
Despite a number of close-calls and scrambles clear from dangerous Lions corners, it was the Reds with the better of proceedings, albeit doing little to deserve all three points.
An injury-time chance fell to Mads Andersen in the box, from 10 yards out, getting on the end of a second ball following a Mowatt free-kick, but the Dane's hooked volley was clawed to safety by former Reds stopper Bialkowski.
It's back to back clean sheets for Struber's side, four points from the six available since the return to play, with Blackburn Rovers next up on Tuesday night.
FULL TIME: Barnsley 0-0 Millwall
Walton (GK), Ludewig, Sollbauer, Andersen, B.Williams, Ritzmaier (Styles), Mowatt, Palmer (Halme), Simoes (Chaplin), Brown, Woodrow (Thomas).
Unused: Collins (GK), Schmidt, Oduor, Bähre, J.Williams.