The trio are all out of contract at Willem II at the end of the season and have opted to pursue the new challenge offered by AZ.
Defensive duo Mathijsen and Jaliens (pictured) had been long-term targets of AZ and have signed four-year deals with the European hopefuls.
Moroccan international Sektioui has decided to follow his Willem II team-mates, with the winger also agreeing a four-year contract.
AZ have a heavy Willem II connection at the club with the Tilburg side's former captain Denny Landzaat, ex-coach Co Adriaanse and old technical director Martin van Geel on their books.
With Mathijsen, Jaliens and Sektioui all arriving at AZ, the Alkmaar club have decided upon the futures of three players already at their disposal.
Former skipper Peter Wijker has been told he will not be offered a contract extension and is free to leave the club.
Joining the defender on his way to the exit door are South American pair Cruz Vicente and Canigia, who have failed to sparkle at AZ.
