The 27-year-old Hornkamp signs a four-year contract in Alkmaar, where he'll provide competition for strikers Troy Parrott and Mexx Meerdink.
Hornkamp scored 10 goals in 14 Eredivisie games for Heracles this season, placing him third in the Eredivisie's top scorer list.
"For me, it's a fantastic start to the new year," Hornkamp said on the club website.
"I actually had the idea of playing for Heracles Almelo until the summer and then making a move. But when AZ showed interest, it quickly became clear to me that I wanted to go for it. It was an easy choice for me."
Hornkamp played for FC Den Bosch, Willem II and Heerenveen before joining Heracles Almelo in 2023. In 62 games for Heracles across all competitions, Hornkamp scored 29 goals.
AZ are currently sixth in the Eredivisie and will appear in the UEFA Conference League's knock-out round after clinching a 14th-placed finish.
