MILLWALL came from behind to win at home in the league for the first time since December and move up to 11th in the Championship table.
Lee Gregory and Steve Morison struck second-half goals after Joey Pelupessy have given the visitors the lead before the break.
Ben Marshall struck the woodwork in the first half as Millwall took the game to their opponents but struggled to test goalkeeper Joe Wildsmith.
Wednesday went in front against the run of play in the 42nd minute when Pelupessy picked up possession on the left before cutting back inside and rifling a shot past Jordan Archer into the far corner.
Millwall were level seven minutes into the second half, Gregory nodding home after Jake Cooper had headed Marshall's free-kick back across goal.
Gregory then forced a save from Wildsmith minutes later after a Millwall move that started in their right-back position.
But the Lions didn't have to wait long for a second in a transformed second-half performance as Morison tapped home Jed Wallace's cross to the far post.
Millwall hadn't won a Championship game at The Den since beating QPR here in their last fixture of 2017, but on the back of their third successive away league win Neil Harris named an unchanged side.
Sheffield Wednesday started the evening five points behind the Lions, but are resilient under Jos Luhukay, having kept seven clean sheets in their previous 10 games.
They started well and hogged possession in the opening 10 minutes without creating any chances, before Millwall then took the initiative.
Marshall was close to his first Millwall goal when he shot right-footed just inside the box, but the ball deflected onto the outside of Wildsmith's post.
Morison thought he had been impeded in the 28th minute as he went to challenge for Wallace's cross, Morgan Fox cutting across him but referee Peter Bankes was unmoved.
Cooper sent a speculative effort from 35 yards just wide and then Gregory was racing onto a pass forward, but with the angle narrowing and two Owls defenders closing in, he couldn't test Wildsmith and fired wide.
Wednesday had rarely threatened and when Morison headed a corner clear it seemed the danger was over three minutes before the break. For a moment it appeared Gregory had a chance to break, but the ball was punted back into the Millwall half.
The Lions players thought Pelupessy was in an offside position but the flag stayed down. Pelupessy still had a lot to do as he cut in on his right foot before smashing his effort past Archer, who had little chance to keep it out.
It wasn't really a fair reflection of how the first half had gone, but Harris would have been pointing out the need for more incision in the final third from his side, after several good moves before the break but the lack of an end product.
Whatever Harris had said at the break worked, as Millwall came out pumped up in the second half. George Saville was certainly in an angry mood, and had several run-ins with Bankes before the referee gave him a warning. And Bankes also spoke to Harris after the Millwall manager entered to pitch to move a Wednesday free-kick back.
That all sparked the crowd, particularly after Wallace's cross appeared to come off David Jones' raised arm in the box. It looked like a penalty, but Millwall soon got the goal their play deserved, a textbook set-piece move and Gregory had his eighth of the season.
Millwall probably don't get enough credit for the quality of the football they are capable of playing. There was a lovely flowing move after an hour, Wallace and Morison combining with Mahlon Romeo before Shaun Williams sent Saville scampering away. He tucked a neat ball inside to Gregory, but Wildsmith palmed away the shot for a corner.
It was little surprise when Millwall added a second. Jordan Thorniley stayed down after a challenge for the ball with Wallace on the right. Millwall's winger kept his head to give Morison a simple task. The game was stopped for nine minutes after the goal, before Thorniley was stretchered off.
That disrupted Millwall's momentum, and it was the visitors who went close to an equaliser when Jack Hobbs volleyed just side with Archer beaten.
There were 13 minutes of added time and Harris brought on Fred Onyedinma for Marshall to add pace on the break.
Substitute Lucas Joao was inches away from making it 2-2, but saw his header go wide of the far post from a corner.
It was Millwall who had the last chance, but Morison couldn't beat Wildsmith with a left-footed shot from the edge of the box.
Image: Millwall FC