The news comes hours after Arsenal confirmed the signing of South Korea captain Park Chu Young.
In the aftermath of Sunday's humiliation, Arsene Wenger was expected to beef up his defensive options and with the capture of the 6'6" German giant Arsenal will be more sturdy at the back.
The aerial presence of Mertesacker is a welcome addition although his lack of pace could be a problem against speedy forwards considering Arsenal's high line of defence.
Mertesacker is a good reader of the game and is very disciplined.
Arsenal's youngsters can learn a lot from him and they need his wise head to guide them past this torrid period.
His organizational skills at the back and experience and pedigree at the International stage will help Arsenal plug the leaky defence that conceded eight goals at Old Trafford last weekend.
Mertesacker is the second signing of the day for Arsenal. Wenger has finally showed his hands in the transfer market, rather forced to show, and more signings are expected to follow on the deadline day.
The news is also out that Fulham have signed Bryan Ruiz possibly as a replacement for Clint Dempsey, who is rumored to be on his way to the Emirates stadium.