After a drab first half, Hibs opened the scoring on 52 minutes when Paul McGinn’s fine cross was headed in powerfully by Christian Doidge
Doidge then somehow failed to make it 2-0 when he scooped the ball over the bar from three yards from Jackson Irvine’s set-up.
It was then the Steelmen’s turn to miss a great chance when Allan Campbell played Devante Cole clean through on Matt Macey but the keeper saved.
Hibs then looked to have clinched the win on 79 minutes when Irvine – in a suspiciously offside position – touched in a Nisbet effort.
But Motherwell pulled one back out of the blue when Ricki Lamie lofted the ball back into the danger area from the back left edge of the penalty area and it crept in at the far post.
Remarkably, the Steelmen then netted an 89th minute equaliser when Stephen O’Donnell’s fine cross from the right was bulleted in by Tony Watt.
Hibs looked the more likely side in the first period of extra time but Motherwell came agonisingly close to going 3-2 up on 113 minutes when Steven Lawless was inches away from turning in O’Donnell’s fine delivery from the right.
The Easter Road side were reduced to 10 men in injury time in extra time when Alex Gogic was shown a second yellow card after tustling with Declan Gallagher.
Players from both sides squared up to each other after the visitors’ Nathan McGinley had fouled Hibs’ Melker Hallberg.
Level at 2-2 after 120 minutes, the game went to a nerve shredding shootout.
Boyle, Cole and Hallberg all netted impressively so it was 2-1 for Hibs before there was agony for ’Well’s Mark O’Hara when he sent his effort over the bar.
After McGinn put Hibs 3-1 ahead, ’Well suffered further disappointment when Steven Lawless shot against the right post.
Although Nisbet’s effort was saved well by Kelly and Long scored to make it 3-2, Porteous struck home the decisive spot kick.