Argyle are now seven games without a win, while Chesterfield clinched their third League Two victory in a week.
The 30-year-old former Port Vale forward smashed the winner in the 66th minute on Saturday after Jack Lester chested down a cross from fellow substitute Luis Boa Morte.
The introduction of the former Portugal international at half-time had the desired effect as he sent a 30-yard shot just wide and then missed the target with a brilliant shot on the spin before setting up the winner.
Argyle should have made the most of their first-half advantage, when they had a shot cleared off the line and hit the bar.
Alex MacDonald sparked the best Argyle spell in the 35th minute, forcing visiting keeper Tommy Lee to go full stretch to keep out a low drive.
From the resulting corner, Richards headed off the line as home central defender Curtis Nelson sent his header towards goal.
Seconds later, with Argyle piling on the pressure, Andres Gurrieri's goal-bound shot was headed up and on to the bar by Spireites skipper Sam Hird.