The design will boast a 120m-tall tower, described as a 'model of sustainable development', which will be surrounded by a large public plaza. There are also plans to build a new light-rail link under the site to enhance access.
An amphitheatre will 'embed' the square into the city, forming a visual link between City Hall on Fourth Avenue and the new People's Pavilion opposite.
Commenting on the project, Foster said: 'Our scheme will combine culture, commerce and community, providing a new civic pace for Seattle. It will create a strong sense of place and a new urban destination for the city that is sensitive to the civic centre masterplan.
He added: 'Our ambition is to create a lasting legacy that will not only lift the fortunes of the area but, crucially, will establish a benchmark for sustainable architecture for future generations.'
The project is expected to complete by 2010.