The expansion of the Sixfields store will almost double the size of the supermarket from 74,000 sq ft to 127,000 sq ft.
The plans were given the go-ahead by the West Northamptonshire Development Corporation.
The store will be re-built and refurbished, including a mezzanine first-floor housing a restaurant.
Christian Wakelin, from Sainsbury’s, said: “We’ll now need to finalise all the legal agreements before looking at our timescales for starting the construction works.”
The approval is subject to the signing of a Section 106 agreement for Sainsbury’s to contribute towards improvements to the highways and Northampton town centre, as well as to sustainable transport initiatives.
As part of the development plans, Sainsbury’s has agreed to invest £217,000 in the town centre.
