The firm will shortly submit a planning application to expand the Orchard shopping centre, doubling retail space.
South Oxfordshire District Council will work closely with Hammerson to ensure that the new scheme benefits the community.
Initial ideas for the site include a 18,580 sq m retail-led scheme, with an anchor department store and a mix of bars, cafe and restaurants.
A hotel could form part of the development, along with a library and homes.
The planning phase will take a year and construction could start in 2012, with late 2013 or early 2014 a realistic timescale for new units opening.
Simon Betty, development executive for Hammerson, said: “The current Orchard Centre, which we have managed for four years, is 100% let and trading well and we are confident in Didcot as a location to invest in.
“We have chosen Didcot because of its enviable location and positive attitude.
“This scheme will put Didcot on the map as a great place to shop and socialise.”