Giroud, 25, scored 38 goals in 69 games during his spell with Montpellier HSC, and Koscielny has backed his fellow Frenchman to reproduce his form in England.
"I am happy [he has joined the club] because he played very well at Montpellier. I played with him at Tours and he was a great player then - now he is even more confident and scores a lot of goals. I know his qualities and he will give a lot for us," he told the club's official website.
"I know he has the physical quality to play in this league. For a striker it is [about] confidence and I am sure he is working hard to start well with Arsenal. He is tall, like Marounane Chamakh and is also good with his back to goal. I think if we play with technical guys he will be happy, and in the box he is a good finisher."
Arsenal open their Premier League campaign at home to Sunderland on Saturday.