The victory was in doubt down the stretch with Detroit pulling within one with 44 seconds remaining on a layup from Brandon Knight.
Minnesota held the Pistons scoreless the rest of the way while making 5-of-6 from the foul line -- three of those coming from Ridnour.
Nikola Pekovic and J.J. Barea led Minnesota with 20 points each in what was the Timberwolves' third win in four outings.
Knight had 25 for the Pistons, who have lost 15 out of 17.
Notable deaths of 2014 [PHOTOS]
Arizona Cardinals player formally charged