Hexagonal boron nitride (h-BN) is a promising platform for quantum information processing due to its potential to host optically active defects with attractive optical and spin properties 1. Recent studies suggest that carbon trimers might be the defect responsible for single-photon emission in the visible spectral range in h-BN. In this theoretical study, we combine group theory together with density-functional theory (DFT) calculations to predict the properties of the neutral C2CN carbon trimer defect. We find the multi-electron states of this defect along with possible radiative and nonradiative transitions assisted by the spin-orbit and the spin-spin interactions. We also investigate the Hamiltonian for external magnetic-field and ground-state hyperfine interactions. Lastly, we use the results of our investigation in a Lindblad (or Gorini–Kossakowski–Sudarshan–Lindblad) master-equation model to predict an optically detected magnetic resonance signal and the g2(τ ) correlation function. Our findings can have important outcomes in quantum information applications such as quantum repeaters used in quantum networks and quantum sensing.