"His knowledge and technical expertise will undoubtedly be a great asset to the team in our continued push to restore Williams to the front of the grid," said deputy team principal Claire Williams.
The South African worked previously at Sauber, Renault and Lotus before joining Ferrari in 2013. His departure from the Italian team was announced last season.
Former champions Williams finished fifth overall last season but are undergoing a technical revamp following the departure of engineering head Pat Symonds. Canadian rookie Lance Stroll and Brazilian veteran Felipe Massa are the 2017 drivers.
Former Mercedes technical head Paddy Lowe is expected to join, once his 'gardening leave' from the world champions is completed.
Aerodynamics is in the spotlight this season with the introduction of new regulations that will see cars cornering faster on much bigger tyres.
