After Brescia were relegated to Serie B in the summer of 2005, Castellazzi joined Sampdoria on a free transfer, signing a reported two-year contract.

[5] After the injury during 2009–10 season, Castellazzi lost his starting place to Marco Storari, and in February 2010, it was rumoured he had signed a pre-contract with Inter Milan.

During the 2010–11 season, Castellazzi played in all three major competitions – Serie A, Champions League and Coppa Italia – due to injuries afflicting the first-choice Júlio César.

He debuted for the Granata on 14 January 2015 in the Coppa Italia against Lazio, replacing Cristian Molinaro after the expulsion of Daniele Padelli.

[10] An experienced shot-stopper, Castellazzi was known for his professionalism and consistency as a goalkeeper, as well as his excellent reflexes and ability between the posts, which enabled him to produce spectacular and decisive saves.