Recent geochronology carried out by Geoscience Australia and the Geological Survey of South Australia as part of the Onshore Energy Security Program in the NE Eyre Peninsula provided the first direct dating of Mesoarchean crust in the Gawler Craton. Four samples of the Cooyerdoo Granite yielded crystallisation ages of c. 3150 Ma, demonstrating that at least parts of the Gawler Craton do indeed have a Mesoarchean basement and providing new insights into the tectonic evolution of the craton. This article provides a formal definition and description of the Cooyerdoo Granite. The Cooyerdoo Granite represents the first Mesoarchean crust outcropping in the Gawler Craton, and adds a further 600 million years to the geological history of South Australia. It also provides the first evidence of possible metamorphism in the northern Eyre Peninsula at c. 2500 Ma, although the structural nature of this deformation is uncertain due to likely rotation during the Kimban Orogeny. The Cooyerdoo Granite is a calcalkaline I-type granite and was derived from the melting of 3250 Ma granitic crust with TTG affinities. It underlies the Middleback Range, where it is basement to the Hutchison Group and extends westwards in the subsurface to the Kalinjala Mylonite Zone, and may underlie the South Australian Heat Flow Anomaly.