The Eredivisie champions have reached a verbal agreement with AZ and are expected to complete the Iceland star's transfer in the coming days
The Amsterdam giants have long been in talks regarding the transfer of the young attacker, but initially refused to increase their €3.5m offer, with AZ holding out for €5m.
Nevertheless, both teams have now agreed terms after intensive negotiations in the past few days.
The 21-year-old is ready to pen a contract until the summer of 2015, and is expected to sign a deal upon completion of his medical on Monday.
Sigthorsson rose to prominence when he scored five goals in one game against VVV in the 2010-11 campaign and had also caught the attention of clubs such as PSV, Newcastle United, Sunderland and Everton.