Liverpool City Council has given the green light to the plans which will add 10,960 sq m to the centre, taking it to a total of 44,500 sq m.
The redeveloped centre will include a double-height mall on the site occupied presently by the market, which is to be relocated. A catering area will be added to the first floor, overlooking Clayton Square.
Work is expected to begin on the project at the beginning of next year and be completed by spring 2013.
Land Securities built the original St John's centre in 1969, which is Liverpool's largest covered shopping centre and home to 105 retail units.