The 25-year contract will involve the design, installation and maintenance of about 46,000 street lights.
Work will include approximately 34,000 new street lighting columns and 11,000 signs and bollards during a five-year capital investment period, as well as the ongoing maintenance of all existing and new equipment.
Balfour will invest equity of approximately £6.5m into the concession.
Chief Executive, Ian Tyler, said: “We are delighted to have reached financial close for this contract. We look forward to a long-term partnership with Northamptonshire County Council to transform the county’s street lighting.”
Balfour currently runs PPP street lighting deals in Cambridgeshire, Coventry, Sunderland, South Tyneside and Derby.