Once complete, Atlantic Shed will provide an extra 7,200 sq m (668,000 sq ft) metres of storage for the port, mainly for the steel industry.
Associated British Ports said the investment meant the site would be able to meet the increasing demands of its customers.
It said the development would support the future growth of the port.
Work has started on a £2.7m project to redevelop part of the Port of Newport to increase cargo volumes.