Police said they had cleared the Oracle centre nearby restaurants and closed nearby roads.
Specialist Hazardous Material Teams are now searching the area as a precaution.
A cordon has been put in place and officers are working to establish the nature of the item.
Some shop managers have sent their staff home in a signal it could be a while before the police allow the centre to open its doors again.
People in nearby buildings have been told to stay indoors and traffic is being diverted away from the area.
Shoppers have complained they are not being allowed back to their cars but there is no indication when the police cordon will be lifted.
Road closures are in place in Bridge Street, Gun Street, Minster Street and Yield Hall Place.
A Thames Valley Police spokesman said: “Our officers remain at the scene at the Oracle shopping centre, Reading as we deal with a suspicious item.
“We’d like to thank everyone for their continued patience as we deal with the incident.”