The Sweden international's deal was set to expire next summer but the new terms will see him stay at the Hawthorns until 2016.
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“This is great news for the club and shows we’re heading in the right direction," Clarke told West Brom's official website.
“We’ve had a productive summer in terms of bringing new players in but it’s equally important that you hold onto your best players, and Jonas certainly fits into that category.
“He’s a tough, uncompromising central defender who knows all about the Premier League and the opponents he’s up against week in, week out. Jonas is also a very important character and leader in our dressing room.”