Siem de Jong and Daley Blind put the visiting Amsterdam side ahead in the first half, and despite Markus Henriksen pulling one back for the hosts, de Jong extended the lead before the hour mark.
Altidore scored in the 74th minute, latching onto a long ball and getting his effort off and under the arm of keeper Kenneth Vermeer just before Niklas Moisander slid in. It was the last goal of the game, as AZ could not find a way back in against the Ajax defense.
AZ is currently 14th in the 18-team Dutch Eredivisie, just one point above the relegation playoff place. Altidore has scored nearly half of the team's league goals, accounting for 18 of AZ's 38 tallies. He has five goals in his last six games across all competitions, and he'll take that form into the upcoming USA World Cup qualifiers against Costa Rica and Mexico.