Well emphatically ended Dundee United's hopes of retaining the cup with a comprehensive victory at Fir Park.
Impressive finishes from Jamie Murphy, Chris Humphrey and Francis Jeffers secured what will be only Motherwell's second Scottish Cup semi-final since winning the competition 20 years ago.
The home side were on top from the start of the replay and always looked like securing a Hampden date with St Johnstone on April 16 after Murphy netted in the eighth minute.
Humphrey and Jeffers, with his first goal at Fir Park, added to the lead either side of half-time as Motherwell knocked United out of the tournament for the first time in four attempts since their epic final triumph over the Tayside men in 1991.
Motherwell's second cup victory over United this season was achieved largely without captain Stephen Craigan, who suffered a hip knock while playing the full 90 minutes of Northern Ireland's goalless Euro 2012 qualifier against Slovenia 24 hours earlier.
However, left-back Stevie Hammell recovered from a hamstring strain to allow Gavin Gunning to line up alongside Shaun Hutchinson in central defence, and they comfortably kept out a United side who had Sean Dillon and Craig Conway back in the team.
Motherwell took less than a minute to open the scoring in the 2-2 draw at Tannadice and they again started the better side with Steve Jennings pulling the strings.
They struck first again when John Sutton flicked the ball on following goalkeeper Darren Randolph's free-kick to find Murphy in yards of space in the inside-left position, and he drilled the bouncing ball into the far corner from 18 yards.
United were soon almost caught out in the same area after a quick Motherwell break with Murphy's reverse pass finding the run of Jeffers, who saw his drilled effort pushed wide of the near post by Dusan Pernis.
Shaun Hutchinson met the resulting corner from Hammell but headed wide.
David Goodwillie threatened to get in behind the Motherwell defence down the left flank on a couple of occasions but holders United were finding it difficult to get forward in numbers.
The visitors, who had Jon Daly at centre-back, began to impose themselves more but Murphy continued to cause problems as he cut inside Keith Watson, forcing Pernis into a decent save from 25 yards before shooting well over from a better position.
Motherwell moved two ahead in the 36th minute. Jeffers collected Hammell's throw and laid the ball back to the left-back, whose cross found Sutton. The striker's header fooled Paul Dixon and fell into the path of Humphrey, who cushioned a first-time volley into the top corner from 10 yards.
United boss Peter Houston immediately made a tactical switch, removing midfielder David Robertson and bringing on left-back Barry Douglas to allow Daly to move up front.
Prince Buaben and Douglas were off target from long range in United's only two first-half efforts and Tom Hateley curled a free-kick just past the post at the other end.
Murphy had the first two openings of the second half, his touch letting him down after a brilliant run from Gunning, before volleying just wide from 25 yards.
Goodwillie had a penalty claim rejected by Calum Murray when Jennings nudged him off the ball, although the striker looked keen to go down.
Dillon soon saw a deflected shot bobble just wide of Randolph's post but Motherwell were soon back on the attack with Sutton heading Hateley's cross at Pernis.
And Jeffers extended their lead in the 63rd minute with a superb piece of strike play, nutmegging a defender 25 yards out before drilling the ball into the bottom corner.
Motherwell saw out the match comfortably and Craigan came on for Hammell with five minutes remaining with the home fans in the 8,337 crowd already celebrating.
Murphy should have finished off a brilliant pass from Jennings but took a poor first touch and Randolph made his first save of the game in the last minute when he touched over Scott Robertson's long-range strike.