Giroud has been impressed with the improvements Koscielny has made in the intervening years but believes there is still more to come from him.
"I'd say he's one of the best in the Premier League," the striker told Arsenal Player. "I think he can continue to [get even better].
"He's very respectable and serious in his work. You can trust him, and his behaviour on the pitch is important. I like when a guy concentrates and is serious.
"He is like a leader on the pitch, by his behaviour. He is important for the national team now, and he is a very important player for Arsenal. He is a very good player and a very good guy."