Mertesacker and Rosicky's new contracts were announced on Tuesday and both have been influential figures in Arsène Wenger's squad this season.
Koscielny's partnership with Mertesacker has been the cornerstone of Arsenal's challenge for honours and he is delighted the Germany defender is here to stay.
"For Per I'm happy," Koscielny told Arsenal Player. "It's his third season here and he signed a new contract. I think he gives everything for the Club, it's a man who is respected by all of his team-mates.
"He's a German so he's professional and it's very important to have a partner like him in the squad. He tried to give himself for the team and when the other players see that, they want to do the same.
"Tomas Rosicky, you know his nickname, little Mozart, and I think he's a player who is very important for us," added Koscielny.
"When he's fit, it's amazing, he can make the difference alone. He scored so many important goals. Like the last one where he played one touch with Santi, Jack and Olivier. He has the technique to play and to give for the team and to make the difference."Copyright 2017 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source