Delage had been due to join Polo Ralph Lauren as president of its Asia Pacific division on 1 September. However, he has decided to join De Beers instead.
He previously spent 16 years at Louis Vuitton Moet Hennessy, where he was most recently president, Asia Pacific, for Louis Vuitton.
De Beers has been criticised over the eviction of bushmen from their homes in Botswana where the group was exploring for diamonds. The firm claimed it has benefited the country by funding its infrastructure.