Simon Davey's strugglers have already beaten Doncaster twice and Sheffield Wednesday, but Beattie's first-half strike and a late penalty were enough to kill off the brave Oakwell outfit.
Both sides played for an hour with ten men after United veteran Ugo Ehiogu and Barnsley's Brazilian Anderson Silva traded headbutts.
Barnsley looked for an early breakthrough but United keeper Paddy Kenny held onto Martin Devaney's stinging drive at the foot of the post.
And when Bobby Hassell whipped in a super cross, new loan signing Simon Whaley controlled and spun but his shot hit team-mate Iain Hume.
Beattie headed wide at the other end after impressive work by Stephen Quinn, who produced a carbon copy cross moments later aimed at Anthony Stokes.
Quinn was involved in the 20th minute when United took the lead. The young Irishman darted down the right, shrugged off Rob Kozluk's challenge and fed Danny Webber.
Webber's sweet turn in the box wrong-footed Hugo Colace before slipping a neat pass to Beattie, who slotted in his emphatic shot.
Seconds later, Beattie was denied a second when he smashed a super shot but Barnsley keeper Heinz Muller matched it with a stunning save.
But the game exploded in the 32nd minute when Ehiogu and Anderson were red carded.
The pair had clashed moments earlier and this time squared up head to head.
Ehiogu appeared to headbutt the Barnsley man who retaliated and ref Andy D'Urso sent the pair off.
Barnsley lost striker Hume three minutes after that following a hefty challenge from former Barnsley defender Chris Morgan who earned a booking.
Barnsley responded hunting for an equaliser but when Colace crashed in a half-volley, Kenny produced a fabulous save and Jamal Campbell-Ryce crashed a shot just off target.
Kenny made another strong save to deny the home side an equaliser when former United defender Kozluk raced in and hit a fierce shot from 12 yards.
Five minutes from the end there was controversy when an assistant more than 50 yards away flagged for a penalty.
Davey was sent off for his protests and Beattie calmly slammed in the spot-kick before Barnsley's consolation - a header from Kayode Odejayi.