Chesterfield's survival hopes continued to fade as Scunthorpe came from behind to take the three points.
Craig Westcarr put the home side in front with a 10-yard finish.
Jordan Robertson levelled when he converted a cross from Conor McAleny, who then laid on the pass for Josh Walker to sweep the Iron in front.
Robertson doubled his personal tally with a 20-yard curler before Andy Barcham slotted home to round off an emphatic Scunthorpe victory.
The rock-bottom Spirites are now 10 points adrift of safety in League One, while Scunthorpe have moved five points clear of the drop zone.
VIEWS FROM THE DRESSING ROOM
Chesterfield manager John Sheridan tells BBC Radio Sheffield:
"It's very disappointing and I can't accept that performance.
"I'll hold my hands up at whether I'm picking the right team or if I'm doing the best job I can.
"So I'll hold my hands up because when you see a performance like that I totally understand everyone's reaction.
"Listen I won't be going anywhere, I'm going to carry on. I'll take what is thrown at me and there's still is a chance of getting out the position we're in."
Scunthorpe United manager Alan Knill told BBC Radio Humberside:
"What we did really well today was defend when we had to defend and then when we had the chances put them in the back of the net.
"In the second half I thought we were very good. It's been an outstanding month for us and the players deserve the rewards.
"When you work as hard as they have since Christmas you can see it in our form and the way they play.
"I thought the whole back four and the goal-keeper were excellent again but the team as a whole just functioned really well."