The construction giant will develop the unit, which will be used by out-patients and in-patients, on a six-and-a-half acre site at Ayrshire Central Hospital, in Irvine.
The building, which will have 206 single en-suite bedrooms, is being funded through the Scottish Government's non-profit distributing model.
Work will begin on the project next month, with the facility expected to be operational by summer 2016.
Meanwhile, Kier Construction is to build a new education campus in Kilmarnock for East Ayrshire Council.
The £35million project will result in the merger of the Kilmarnock and James Hamilton academies on one campus, alongside the merged Silverwood Primary and Early Childhood Centre.
A centre of excellence for Gaelic language development will also be housed on the site, subject to ministerial approval.
Kier won the project under the hub system, a public-private approach to infrastructure projects.