A quantum coherent spin in hexagonal boron nitride (hBN) at ambient conditions has been demonstrated, offering a promising platform for quantum networks and sensors. The study identifies a carbon-related defect with a spin-triplet electronic ground-state manifold, enabling quantum coherent control under ambient conditions. The spin coherence is governed by coupling to a few proximal nuclei, which can be decoupled to prolong coherence times. The research shows that the spin coherence time can be extended beyond 1 µs at room temperature without a magnetic field, indicating protection from nuclear decoherence. The defect exhibits a spin-triplet ground state with a zero-field splitting of 1.96 GHz, and the spin coherence is enhanced using dynamical decoupling protocols, achieving a spin-echo coherence time of ~200 ns. The results suggest hyperfine coupling to a few inequivalent nitrogen and boron nuclei. The study also reveals that the defect has a low-symmetry chemical structure, which is crucial for its quantum coherence properties. The findings highlight the potential of hBN as a scalable platform for quantum spin-photon interfaces, with applications in quantum repeaters and sensors. The research provides insights into the microscopic structure of the carbon-based spin-triplet defect and its potential for quantum technologies. The study demonstrates the feasibility of room-temperature quantum spin control in hBN, offering a new platform for quantum information processing and sensing.A quantum coherent spin in hexagonal boron nitride (hBN) at ambient conditions has been demonstrated, offering a promising platform for quantum networks and sensors. The study identifies a carbon-related defect with a spin-triplet electronic ground-state manifold, enabling quantum coherent control under ambient conditions. The spin coherence is governed by coupling to a few proximal nuclei, which can be decoupled to prolong coherence times. The research shows that the spin coherence time can be extended beyond 1 µs at room temperature without a magnetic field, indicating protection from nuclear decoherence. The defect exhibits a spin-triplet ground state with a zero-field splitting of 1.96 GHz, and the spin coherence is enhanced using dynamical decoupling protocols, achieving a spin-echo coherence time of ~200 ns. The results suggest hyperfine coupling to a few inequivalent nitrogen and boron nuclei. The study also reveals that the defect has a low-symmetry chemical structure, which is crucial for its quantum coherence properties. The findings highlight the potential of hBN as a scalable platform for quantum spin-photon interfaces, with applications in quantum repeaters and sensors. The research provides insights into the microscopic structure of the carbon-based spin-triplet defect and its potential for quantum technologies. The study demonstrates the feasibility of room-temperature quantum spin control in hBN, offering a new platform for quantum information processing and sensing.