Marr College in Troon will undergo a comprehensive refurbishment and expansion programme.
The original listed Grade B building will be refurbished and a new wing will be built over three storeys, connecting it to the rear of the original building over all floors.
A single-storey sports centre will also be built for both pupils and the wider community out of school hours.
All works to Marr College will be carried out while the school remains operational and work is due to start in the third quarter of next year.
Kier is also due to begin work on the site of the new Ayr Academy in the third quarter of next year.
The secondary school, which is currently located in the town’s Fort Street, will relocate to the new Craigie Estate when the new build is complete in spring 2017.
The new school, with integrated sports block, will initially cater for 1,000 pupils, with the potential to increase that number to 1,300 following completion of housing developments within the catchment area, effectively doubling the current roll of 617.
Managing director of Kier Construction’s Scotland & north-east England business, Brian McQuade, said: “There is no doubt that these projects will provide a substantial boost to the local economy.
“The significant financial investment, combined with the employment opportunities that the developments will create for locally based suppliers and subcontractors, is great news for the people of South Ayrshire.”