De Cler has never looked back since being moved on by Ajax in the summer of 2002, proving himself to be an integral figure for AZ.
The 25-year-old has played a key role for AZ as they have mixed it with the big players in De Eredivisie and the club have rewarded him for his efforts.
De Cler's previous deal was due to expire in the summer of 2005, but he has now added a further three years to his commitment to AZ.
AZ director of football Martin van Geel was pleased to have extended the contract of the talented defender.
"We are satisfied with his development and are pleased to have got a quality Dutch left back tied to us," van Geel told the club's website. "Especially when you look at the market."
AZ have also extended the contract of their talented young goalkeeper Theo Zwarthoed.
Zwarthoed's contract was due to expire next summer but he has now committed his future to the club until 2007.
