Mertesacker has been a rock for Arsenal this season, making 14 starts across all competitions en route to helping the Gunners top the Premier League, per ESPN FC.
Wenger revealed in Friday’s press conference, as reported by Sami Mokbel of the Daily Mail: "We are in the process of extending Per Mertesacker's contract."
Mertesacker was badly missed in Arsenal's last outing—a 1-0 defeat to Manchester United, which saw the Red Devils take advantage of their opponents’ lack of height at a corner.
The Germany international missed the game due to illness, per Sky Sports, and his absence led to only Arsenal’s second league defeat of the campaign.
Mertesacker provided yet more evidence of his fine form on Tuesday at Wembley, when he scored the only goal as Germany downed England in a World Cup warm-up encounter.
At 29, he is approaching an age when Arsenal begin to offer shorter contracts to their players, so there is always a fear that policy may lead individuals to seek more secure futures elsewhere.
Will Arsenal win the league?
However, Wenger sounds confident he will retain Mertesacker, who has formed a solid bond with Laurent Koscielny at the heart of Arsenal’s defence.
The Gunners have conceded 10 goals in 11 league outings and appear set to advance to the knockout rounds of the Champions League.
Wenger only has Thomas Vermaelen as natural cover for the pair, so the importance of keeping Mertesacker is heightened. The expected new deal will protect Arsenal’s short-term future.