The Germany international has featured in 16 of the club's games this season, helping them to top both the Premier League and Group F in the UEFA Champions League.
Speaking on Friday, Wenger said talks were ongoing to secure an agreement with the 29-year-old.
"He has one and a half years to go, so that's a long time," he stated.
"We are in the process, yes, of talking."
Mertesacker, who scored in Germany's 1-0 win over England on Tuesday, moved to the Emirates Stadium from Werder Bremen in 2011.
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