The 35-year-old is expected to seal a £3 million move to Watford, where he spent two seasons on loan earlier in his career, now that the Johnstone deal is completed.
Richardson actually spent time on loan at the Hawthorns during his time with Manchester United when he helped to keep the Baggies in the Premier League with a loan spell that saw him score three goals in 12 matches for the club. However, failure to reach the Premier League after their all or nothing attempt has left them unable to raise the funds, allowing Midlands rival and recently relegatedWest Bromwich Albion free to swoop.
Head coach Darren Moore added: "Sam is young and determined to be the best keeper he can possibly be - he's hungry to do that with us at Albion".
Johnstone leaves Manchester United following the acquisition of goalkeeper Lee Grant from Stoke on a two-year contract.
It remains to be seen if Re-Coker will feature in any pre-season friendlies with West Brom or whether he is just training with the club to stay fit as he looks to find himself a new club.
He joins the Baggies for an undisclosed fee on a permanent four-year-deal.
"I'm happy to be here", he declared.
The club also announced the sale of goalkeeper Sam Johnstone to West Brom. "This was an opportunity that I couldn't let pass and I'm thrilled to become a part of such a historic club", Grant told the club's website. I've played Championship games and won a promotion from League One with Preston but always seemed to end up back at Manchester United or going somewhere else.