Martelli announced the move via Twitter.
Martelli was fired by Saint Joseph's in March after 34 years with the Hawks, 24 as a head coach and 10 as an assistant. During his time at the helm, St. Joe's went to the NCAA tournament seven times and made six NIT appearances. They won four Atlantic 10 regular-season championships under Martelli, including the 2004 team that reached the Elite Eight behind the star backcourt of Jameer Nelson and Delonte West.
Saint Joseph's struggled in Martelli's final three seasons and went 14-19 overall last season.
Howard was hired earlier this month to replace John Beilein, who left for the Cleveland Cavaliers. Howard is expected to retain assistant coach Saddi Washington, who was an assistant under Beilein, and former NBA player Howard Eisley is also in the mix to be on Howard's staff.