The acquisition also ensures Whitbread will be an early investor in the Stratford regeneration project which will see the creation of a vibrant new business centre for London.
Whitbread chief executive Alan Parker said: "Securing this first-class location adjacent to the Olympic Stadium is a great result for us. With our expansion over the next two years Premier Inn will have an unrivalled presence across London with more bedrooms in more locations than any other hotel operator."
John Burton, Director of Westfield Stratford City, added: "Premier Inn is a fantastic addition to the scheme. The hotel will be in a prestigious location adjacent to the International Station. Westfield Stratford City has unrivalled connectivity and the opportunities that offers in terms of catchment are unparalleled.
"Westfield Stratford City will be projected onto a world stage during the Olympic Games in 2012 and we are creating a new metropolitan hub which will transform east London for the wider community, prompting increased investment and business activity in the area for years to come."