The Gunners open their UEFA Champions League campaign with a trip to the German champions on Tuesday.
And former Werder Bremen ace Mertesacker warns that they could not have faced a tougher opener than against Dortmund.
"Dortmund were the best team in Germany last year, they do a good job passing the ball and have great players," said the German central defender.
"It will be difficult for us but I know we have very good players and I'm confident we can get a result.
"Dortmund are the toughest opponent in our group, so we will be very happy if we can take something from this match."
Mertesacker made his Arsenal debut in the 1-0 win over Swansea on Saturday, and he admits that despite his experience it is a test for him playing in the Premier League.
"I have a lot of experience but it is a new level for me to play in England although my targets and goals are high," he added.
"One thing is for sure, I will give everything every time I put on the Arsenal shirt."