The WPP digital agency has been asked to design and build a new global content web platform that will attract companies and potential employees to the company.
Paul English, the global head of marketing said: "Possible’s impressive digital expertise and global capabilities assured us that they were the right partner for us.
"With member firms in over 130 countries across the globe, it is vital that Grant Thornton is able to ensure brand consistency.
"The main objective is to use digital channels in a much more impactful and user-centric way to attract dynamic clients and people to Grant Thornton."
The aim is to create a consistent brand platform across the 130 countries in which it works. The site will be mobile-enabled and will use a more editorial approach.
Justin Cooke, the UK chief executive of Possible, said: "We are passionate about delivering digital products for our clients that make a measurable difference.
"The launch of a new global web environment for the brand will not only align its firms across the world but also provide a platform that will support growth in the Grant Thornton organisation."