Goals from Niall Canavan, Sam Winnall, Jimmy Spencer and David Syers secured the points against a home side depleted in numbers after central defenders Oliver Lancashire and Jack O'Connell were sent off in the second half.
Both teams had the ball in the net during an opening 45 minutes largely bossed by Rochdale, but both Winnall for the Iron and Matt Lund for the home side saw their efforts ruled offside.
The visitors took the lead five minutes before the break. Dale could only partially clear Terry Hawkridge's corner from the right and when David Mirfin headed the ball into the mix Canavan reacted quickest and poked home from close range.
After the interval Rochdale tested Iron goalkeeper Sam Slocombe with efforts from Matt Done and Bastien Hery but their comeback attempts were hindered when skipper Oli Lancashire was dismissed in the 67th minute for a second bookable offence.
And Rochdale were down to nine men when Jack O'Connell hauled down Jimmy Spencer as he bore down on goal. Referee Tony Harrington sent the Dale defender off and awarded a penalty to Scunthorpe which Sam Winnall converted.
Scunthorpe made their numerical advantage count in the closing stages as Spencer thumped home in the 86th minute and Syers added a fourth in time added on.