Sparks, 26, was acquired in July by the Golden Knights from the Toronto Maple Leafs in exchange for a 2020 fourth-round pick and David Clarkson.
Sparks was technically the longest-serving Maple Leaf at the time of the deal. He was selected by the club in the seventh round of the 2011 NHL Draft, but only suited up for 37 games over the course of his career in Toronto.
Sparks initially stole the backup position behind Frederik Andersen last season, but struggled to hold onto the role, finishing with a .902 save percentage and a 3.15 goals-against average in 20 games.
Johnston said the Golden Knights also placed netminder Oscar Dansk on waivers Saturday.