The centre-half had spent the previous five years at the German club, whose closest rivalry was with Hamburg - a club 60 miles away.
Mertesacker said: "This is my first special game for Arsenal. A lot of people have been telling me about it, how massive it is, and what a special experience it is and I am looking forward to it.
"It is an especially important game for Arsenal and for me. There is nothing similar to it in Germany. You cannot compare Bremen against Hamburg because they are not quite in the same city, so the London derby is bigger."