The construction and engineering company has been appointed as contractor on the 160,000 sq ft office in Mosley Street in Manchester city centre, with work set to begin immediately.
The £80m block on the site of Century House, Sussex House, Bennett House and Clarendon House is scheduled to complete in January 2017.
The scheme is being delivered by developer Mosley Street Ventures, a special purpose vehicle controlled by Fred and Peter Done.
Two St Peter's Square is designed by SimpsonHaugh & Partners and will include 5,000 sq ft of ground floor leisure space and car parking for 72 vehicles.
Tom Higgins, head of Laing O'Rourke in the North, said: "We have a strong working knowledge of the St Peter's Square heritage development area, through our delivery of the Town Hall Complex transformation and Second City Crossing. This, combined with our use of offsite techniques and modern methods of construction, means we'll be able to deliver a high quality scheme far more efficiently and with minimal disruption to the city centre.
"Once the project is completed it will be one of the most prestigious office buildings in Manchester. The building has been designed to complement the changing face of St Peter's Square being sympathetic to its history and architecture."