Sonko had fallen out of favour with Tony Pulis and made just one senior appearance last season.
“He’s a big, strong and very powerful defender. I’ve followed him for a while and I thought he did a fantastic job at Reading,” Steve Cotterill told the club’s official website.
“He’s been a bit unlucky during his time at Stoke and hasn’t had too many first-team opportunities, although he did spend time on loan at Hull last season. He just needs to get back playing football.
“We’re very thankful to Stoke’s chief executive, Tony Scholes, and to Tony Pulis for their help.”