The agreement, which was announced this morning, relates to the delivery of land, development rights and the provision of infrastructure, utilities and services to Stratford City and the Olympic facilities.
The Westfield project - a 175,000 sq m retail and leisure scheme - is expected to be finished by 2011.
Briefing Prime Minister Gordon Brown at No 10 Downing Street, Westfield chairman Frank Lowy said: "The £1.5 billion Stratford City retail and leisure development has both scale and significance, serving as a gateway to the Olympic Park and delivering real momentum to the regeneration of east London."
Also present at the meeting was London mayor Boris Johnson, mayor of Newham Sir Robin Wales and Olympic Delivery Authority chairman John Armitt.
Mr Brown said: "Development in and around the Olympic Park site will bring real benefits to the people of east London, transforming the local area and bringing billions of pounds of private investment into one of the most disadvantaged parts of the UK.
"Regeneration was at the heart of our bid for the 2012 Olympic and Paralympic Games, so I welcome the excellent progress being made on the Stratford City development."