The Vikings play the Green Bay Packers on Thursday night.
Matt Cassel, who began the season as the Vikings' starter, is out for the season with a foot injury. Before the signing of Harnish, Bridgewater and Christian Ponder were the only two QBs on Minnesota's roster.
Harnish was signed to the Vikings' practice squad on Monday. The Colts drafted him in the seventh round in 2012 and released him at the end of the 2014 preseason. He has never appeared in an NFL game.
Harnish played four years at Northern Illinois. In his senior year, he threw for 3,216 yards, 28 touchdowns and six interceptions.
The Vikings waived wide receiver Rodney Smith to make room for Harnish on the roster.
- Molly Geary