Per Mertesacker joined Arsenal in 2011 from Werder Bremen and his current contract will expire this June. He hasn't made any apperances this season due to a knee injury.
"He has an option that we have taken. There was no negotiation, just an option that we have taken. He's back in training, not with the squad, but he's two weeks away now," said Wenger.
"With the number of games we have it's important that we have all our experienced players back and that everybody is focused to do well.
"We're going through a moment of truth now, January until May is five months to go. You want ideally all your players with big experience back in your squad," the coach told SkySports.