A goal from Shaun Barker looked to have given Rotherham their first win in 10 league games but Watt headed in a leveller in the fourth minute of injury time
But referee Tim Kettle stole the show, sending off Deon Burton after 56 minutes, Lee Williamson after 85 minutes and Colin Murdock at the final whistle.
Rotherham deservedly led at half time after defender Barker punished Barnsley's poor defending with a goal after 32 minutes.
The Millmoor side, who arrived at Oakwell with a dismal record of nine league outings without a win, flashed an early warning when striker Martin Butler burst forward after five minutes but hit his shot was straight at keeper Nick Colgan.
Leading scorer Burton should then have done better with another break forward but, after carving out an opening, he crashed his shot high over the bar.
As Rotherham forced their neighbours onto the back foot Williamson pounced on another opportunity but crashed an ambitious shot over the bar.
Barnsley only had themselves to blame when Rotherham's goal came. Paul McLaren curled in a testing free-kick that caught out the defence and keeper Colgan, leaving centre-back Barker to steal in and volley home from close range.
Burton should have done better when another free-kick from McLaren deceived the home defence, but Burton only succeeded on heading wide.
Barnsley's first chance arrived in first half stoppage time when Jacob Burns lashed a speculative 35-yard shot that bounced awkwardly but ran wide.
It was Burns who might have found a way back for the hosts early in the second half, but he blasted a shot wide after a cross from Martin Devaney.
Fussy referee Kettle left Rotherham down to ten men when he sent of Burton in the 56th minute for his second booking when he dived after a challenge from Watt.
As Barnsley tried to make the extra man count Rotherham almost caught them on the break when new loan signing Paul Evans coaxed a full length save from Colgan with an angled drive.
Barnsley's best chance fell to Marc Richards, who sliced his shot wide after a super ball from Devaney, as the home side tried to make the extra man count.
As Rotherham defended desperately Williamson was sent off for a second bookable offence five minutes from the end.
Millers boss Harford was ordered to the stand with a minute to go, before Murdock became the third visiting player dismissed in the final minute as Watt rescued a point for the Tykes.