Hiwaizi replaces Robin Jeffray, who left in April.
In his new role, Hiwaizi will head Chemistry's planning offering across clients including Orange and Baileys.
The appointment puts an end to speculation that Hiwaizi, who left Crayon in April, was planning to set up his own consultancy.
His departure coincided with the appointment of Jonathan Clark, the co-founder of Clark McKay & Walpole, who took on the newly created role of chairman.
Hiwaizi began his career as the art editor at i-D magazine in the early 90s before founding the direct marketing start-up HHM in 1995. He later helped found Crayon in 2005.
His appointment comes shortly after Chemistry secured the £22 million direct marketing business for Orange. The agency recently snatched the business from Wunderman without a pitch.
Joe Garton, the chief executive of Chemistry, said: "I am delighted that Omaid is joining the agency as he brings a wealth of brand planning, strategic creative development and experience in executing through-the-line communications."