According to Sky Sports, a deal has been agreed between both clubs and the player is all set to arrive in Birmingham tomorrow to complete his medical and other formalities of the move.
The 37-year-old will be on loan until the end of the season.
Dean Smith lost Tom Heaton to an injury earlier this month and Reina will come in as his replacement.
It will be interesting to see how the Spaniard performs during the second half of the season. He had a very good spell in the Premier League with Liverpool in the past.