ROMAN Bednar's sixth goal in just seven league games set Albion on course for a fourth successive win against ten-man United.
The Czech striker broke the deadlock with a deft back-heel in the 13th minute.
The Baggies created enough chances to put this game to bed early and their profligacy created a nervier finale than it should have been.
But sub Gianni Zuiverloon finally wrapped up the points by polishing off a fine 86th-minute move - his second of the season against Scunthorpe.
The Iron's night got worse two minutes later when skipper Cliff Byrne was red carded for denying sub Andy Slory a goalscoring opportunity.
Albion weren't quite at their best but deserve great credit for fighting the fatigue to record a fourth victory inside 11 days.
New loan signing Frank Nouble was handed an immediate Albion debut as Roberto Di Matteo rang four changes from Saturday's win at Plymouth.
The West Ham forward replaced the suspended Jerome Thomas on the left wing while rested trio Gonzalo Jara, Jonas Olsson and Robert Koren earned recalls. Zuiverloon, Abdoulaye Meite and Simon Cox dropped to the bench.
Iron boss Nigel Adkins also made four changes, with Josh Wright, Michael O'Connor, Brendan Moloney and Matthew Sparrow coming in for Grant McCann, Paul Hayes, Marcus Williams and Garry Thompson.
Scunthorpe started the day in 19th place - only four points above the drop-zone - but it was the visitors who threatened first.
Joe Mattock was guilty of conceding possession to Sparrow and, from his lay-off, Sam Togwell rippled the side-netting with a 20-yard strike.
But Albion soon took control and carved out a great opening in the eighth minute.
Bednar threaded Nouble down the left and the 18-year-old composed himself before returning the ball to the Czech striker, who was disappointed to side-foot just over from 15 yards.
Bednar was proving a real handful for The Iron's defence and it wasn't long before he teed up the marauding Mattock, who fired inches wide of the far post from just inside the box.
The Albion pressure continued when the lively Nouble held off Byrne on the edge of the penalty area before seeing his low shot turned wide by former Baggie Joe Murphy.
A goal had been on the cards and, unluckily for Adkins' side, it duly arrived on 13.
Jara sprayed a delicious ball to Koren wide on the right and the Slovenian skipper's inch-perfect cross was sublimely back-heeled home by Bednar at the near post.
Albion looked comfortable but were guilty of taking their foot off the gas in the 25th minute when slack marking allowed Michael O'Connor to find Sparrow, who fired a 20-yard snapshot just wide.
With the Baggies suddenly losing their spark, Scunthorpe grew in confidence as the half progressed and began to knock the ball around in style, albeit without testing Scott Carson.
O'Connor tried his luck himself in the 36th minute when he drilled a low 20-yard effort goalwards from Gary Hooper's ball, but Carson saved comfortably low to his right.
Albion didn't threaten again until six minutes before the break when Murphy had little trouble keeping out Koren's 18-yard shot after a clever one-two with Bednar.
But Chris Brunt gave Scunthorpe much more cause for concern seconds later when he skipped inside Donal McDermott before unleashing a left-footed strike which would have found the far corner had Rob Jones not got in the way.
The Baggies were back on their game and Murphy did well to keep out Nouble's acute drive after he had left Byrne for dead.
Albion took their time getting going again after the break but, in the 54th minute, Murphy was relieved to see two fierce strikes from Graham Dorrans and Gabriel Tamas deflected wide.
Centre-half Tamas then saw a near-post header from Dorrans' corner clip a Scunthorpe head and loop onto the roof of the net.
The Baggies were forcing the issue again and Murphy had to pull off a wonderful reflex save to keep out Brunt's 18-yard strike after a terrific run by Dorrans.
Tamas was soon called into action at the other end when he diverted wide Togwell's volley from just inside the box.
While the game remained at 1-0, The Iron were still very much in it and Hooper wasn't too far over with an ambitious 20-yard drive from McDermott's lay-off.
However, Albion should have given themselves further breathing space in the 65th minute when great interplay by Koren and Bednar put Brunt clear.
But with just Murphy to beat, albeit from a slight angle, the Northern Ireland international dragged his shot wastefully wide of the near post.
Brunt tried to make amends in the 69th minute when he skipped across the edge of The Iron's box before seeing a low drive pushed away by Murphy.
Albion were turning the screw and Dorrans cut in from the left and saw a goalbound shot deflected wide by Sparrow.
But the Baggies had a let off on 74 when they only half-cleared an O'Connor corner. David Mirfin fizzed the ball back into the box and Jones fired wastefully over from 12 yards.
However, Albion finally wrapped up the points four minutes from time - from a move started by Carson.
Bednar and sub Simon Cox combined well down the right to tee up Zuiverloon, who slotted clinically home from eight yards.
Salt was poured into United's wounds two minutes from time when skipper Byrne was red carded for scything down sub Slory 25 yards out and denying the winger a goalscoring opportunity.
Cox, who hit the winner at Plymouth on Saturday, almost added a third in stoppage time but was denied by another fine Murphy save.
ALBION (4-5-1): Carson; Jara, Tamas, Olsson, Mattock; Brunt (Zuiverloon 81), Koren (Cox 66), Mulumbu, Dorrans, Nouble (Slory 74); Bednar. Subs not used: Kiely (gk), Moore, Meite, Thorne.
SCUNTHORPE (4-4-2): Murphy; Byrne, Mirfin, Jones, Moloney (Morris 23); Sparrow, Togwell (McCann 81), O'Connor, McDermott (Woolford 62); Wright, Hooper. Subs not used: Lillis (gk), Hayes, Forte, Raynes.
GOALS: ALBION - Bednar (13), Zuiverloon (86).
BOOKINGS: ALBION - Brunt (foul 22). SCUNTHORPE - Sparrow (foul 45+1).
SENDING OFF: SCUNTHORPE - Byrne (denying goalcscoring opportunity 88).
REFEREE: K Wright (Cambridgeshire).
ALBION STARMAN: Chris Brunt.