The Iron looked set for a third successive home defeat as the Spireites capitalised on two second-half defensive errors by the home side.
Scunthorpe broke the deadlock in the 24th minute when Michael O’Connor curled a deep corner to the far post where centre-back Niall Canavan rose to head his first goal of the season.
The visitors were back on level terms three minutes into the second half when Scunthorpe gifted away possession on the edge of their own box and substitute Scott Bowden rifled in a shot that went in off the far post.
Alan Knill’s men thought they had regained the lead after 65 minutes when Chris Dagnall headed home a corner from O’Connor, only to see it ruled out for a pushing offence. Five minutes later it was Chesterfield who were in front when a howler of a clearance by home keeper Sam Johnstone left an open goal for striker Craig Westcarr to find with a shot from 25 yards.
Chesterfield looked set to end three months without a win until Ryan struck for Scunthorpe in the second minute of stoppage time, drilling a 25-yard shot through a crowded box and into the bottom corner past keeper Tommy Lee.