Adam Armstrong has signed for West Brom on loan from Premier League Southampton
In a grandstand finale at The Hawthorns, Jayson Molumby snatched a 96th-minute winner for West Bromwich Albion to deny Sheffield Wednesday a last-gasp point.
Callum Paterson had fired a 93rd-minute leveller after Adam Armstrong looked to have netted a classy winner on his Albion debut 16 minutes from time.
The win made it 11 unbeaten at home for Albion and took Tony Mowbray's side up to fifth in the Championship.
Armstrong had struck the post after Owls keeper James Beadle denied Torbjorn Heggem in a first half the hosts dominated.
However, the dangerous Djeidi Gassama drew a fine save from Joe Wildsmith at the other end and saw a goal chalked off by a late offside flag in the second half.
Mowbray handed Armstrong his debut while Joe Wildsmith took the number one shirt after Alex Palmer's departure for Ipswich, lining up against the club where he spent the first eight years of his career.
Defender Mason Holgate missed out through illness, with Kyle Bartley returning to the team and captaining the side.
Wednesday skipper Barry Bannan missed just his second game of the season through injury in a major blow for Owls boss Danny Rohl who dropped Paterson and Jamal Lowe to the bench with Nathaniel Chalobah, Michael Smith and debutant Stuart Armstrong coming in.
Albion piled on early pressure and Isaac Price's corner on 19 minutes was knocked down by Bartley, though the unmarked Karlan Grant could only hook wide of the near-post from six yards out.
Heggem's downward header from another Price corner – one of 10 the hosts won in the first half - was clawed away by Beadle, with Armstrong's close-range follow-up thumping the outside of the upright from an angle before the Owls goalkeeper raced off his line to thwart Grant two minutes later.
The visitors nearly stole the lead on a rare foray forward just before the break with Gassama cutting in from the left and firing in a low shot which Wildsmith expertly tipped around his near post in his first action of the afternoon.
Owls boss Danny Rohl was dealt a blow as Di'Shon Bernard was forced off after twisting a knee, with Marvin Johnson coming on and forcing a reshuffle at the back.
Johnson produced a goal-saving block just a minute after the interval as he slid in to deny Grady Diangana's low effort after a slick counter from the hosts and Johnson also sliced a Molumby cross into his own post, with Beadle gathering the rebound, soon after.
The Owls thought they had broken the deadlock against the run of play on 51 minutes as Gassama cut in from the left and drove into the box before curling a low shot inside the far post, only for their celebrations to be cut short by a very late offside flag against Josh Windass in the goalmouth, which had obstructed Wildsmith's view, leaving Rohl irate on the touchline.
The Owls boss replaced Stuart Armstrong and Chalobah with Pol Valentin and Svante Ingelsson, with the Swede blazing over from the edge of the box on the hour as the visitors enjoyed a good spell of pressure.
Albion had appeals for a penalty for handball against Max Lowe from Furlong's cross into the box waved away by referee Tom Nield before Mowbray introduced Tottenham loanee Will Lankshear, Tom Fellows and Mikey Johnston from the bench, with Fellows blazing over from outside the box moments later.
Armstrong eventually bagged his debut goal 16 minutes from time, timing his run to perfection to meet Fellows' cut-back from the right byeline and guide a left-footed volley across Beadle's goal and inside the far post.
Wednesday applied late pressure and Wildsmith blocked substitute Ike Ugbo's half-volley after Paterson's knockdown.
But on 93 minutes the keeper could only parry a low cross from Ingelsson on the left, with Paterson slamming home at the second attempt after his initial effort was blocked on the line.
There was still time for Beadle to superbly tip over Johnston's deflected strike at the other end, but from the resultant near-post corner on 96 minutes by Johnston, Molumby found space to flick a low effort inside the near post through a crowd and spark wild scenes of celebration.
Tempers flared after the final whistle with Bartley shown a red card for violent conduct after appearing to push Ingelsson in the face in a melee.
Jayson Molumby has scored in successive games for West Brom
'Brilliant movement from Molumby'
West Bromwich Albion boss Tony Mowbray told BBC Radio WM: "I thought there were lots of positives. We were very good first-half, total domination, I think they had one shot.
"We had lots of the ball, just couldn't break through. Second-half was more open, they came more into it, but we had some really good chances.
"Adam Armstrong is a good footballer, he's going to score goals. If he plays the next 15 games, he might not quite get to double figures but he'll not be too far off.
"Ultimately the score is 2-1 to us and we deserved to win over 90 minutes but the injury-time equaliser was a kick in the stomach for us and then to score again so soon afterwards is something the lads have been talking about – we don't score many late goals, how do we cause carnage in the box at the end of a game to score a goal and win a tight match.
"It was brilliant movement in the box from Molumby. I felt over the 97 minutes we deserved to win the game. I am pleased for the fans. We just have to keep going."
Mowbray on Bartley's post-match red card: "I didn't know until he came in and said he'd had a red card. It's really disappointing and frustrating for us. I think he's going to be really important for the team. It's going to be a blow if he's going to miss three games."
Sheffield Wednesday boss Danny Rohl told BBC Radio Sheffield: "The second half was a good Championship game from both teams, both teams tried to win it.
"It's hard. I really feel for my players. I think we deserved a point. In the first half West Brom did more for the game, but it was ok for me, today. We changed the game with the subs. A draw would have been fantastic today, and deserved.
"We had the chance to take a point here but at the end we conceded a goal from a set-piece. It's hard to take.
"We believed we could do something. In the first half we had a little too much respect sometimes."
On the disallowed Djeidi Gassama goal: "I never complain about decisions, it is what it is, it's football, but to take a goal back after 16 seconds is a little bit hard to take. It's not really clear for me."
