During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress(1). Many foundational experiments have been performed(2-8), and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention(9-12) since the first demonstration of macroscopic quantum coherence in Josephson junction circuits(2). Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen-vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3 x 10(7) such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.