The Serie A title contenders confirmed the double signings, Portuguese centre-back Rolando arriving from Porto and Croatian youngster Radosevic from Hadjuk Split.
“I am very happy to be here,” Rolando, 27, told the club's official website.
“I wanted to join Napoli because it is a great club with many top players.
“I have spoken before about the passion of the fans and I cannot wait to play at Stadio San Paolo.”
Napoli currently sit second in Serie A, three points off league leaders Juventus.