The Baggies hero hangs up his gloves having left previous club Watford upon relegation from the Premier League at the end of last season.
Leamington Spa-born Foster, 39, spent seven seasons at The Hawthorns – one as a loan player from Blues – and totalled 223 appearances in all competitions for the club.
Having initially been brought in by Roy Hodgson, his replacement Steve Clarke made the deal permanent and Foster's entire Albion career was spent in the Premier League.
After leaving Albion in 2018, Foster spent four years with Watford, three in the top flight.
The former Stoke, Blues and Manchester United goalkeeper also earned international honours and won eight England caps between 2007 and 2014.
He will be involved in the 'Brunty v Mozza' charity fixture at The Hawthorns on Saturday week.
Meanwhile, former Albion striker Andy Carroll, 33, has rejoined former club Reading on a free transfer. He spent the second half of last season with the Baggies.