Former Wales international Knill said: "It has been a tough season for all sorts of reasons but I am delighted to get the chance to move the club forward.
"The mood in the squad is good, we have players with a never-say-die attitude who have the ability to take this club up the league."
Knill made 86 appearances for Rotherham after joining from Glanford Park.
He holds the UEFA A coaching licence, and has coached the youth and reserve sides at Millmoor.