We report a field-induced quantum first-order transition in the quasi-two-dimensional antiferromagnet ${\mathrm{KCoPO}}_4·{\mathrm{H}}_2\mathrm{O}$. Using highly oriented microcrystals with $H\parallel b$, polarization neutron diffraction directly resolves microscopic coexistence between an incommensurate cycloid and a commensurate collinear state on the same specimen, together with a discontinuous jump of the ordering vector from $\mathbf{q}=(\delta ,0,0)$ ($\delta =0.1747$) to $\mathbf{q}=\mathbf{0}$. Magnetization and specific heat show a step and a collapse of the $C/T$ anomaly near ${\mu }_0{H}_c\approx 0.6\mathrm{T}$ with no measurable hysteresis, consistent with strong quantum-fluctuation-induced rounding of first-order signatures in the $T\to 0$ limit. LF-$\mu \mathrm{SR}$ reveals two-component relaxation and loss of the late-time tail around $H_c$, with slow fluctuations persisting to 0.06 K. The $H\text{−}T$ phase diagram exhibits a narrow coexistence region. By the symmetry of space group $Pmn{2}_1$ (Moriya's rules), the Dzyaloshinskii–Moriya vector is constrained predominantly within the $bc$ plane, thereby favoring a spiral with $q\parallel a$. Conversely, XY anisotropy competes with this tendency, producing a near degeneracy between the incommensurate cycloid and the commensurate collinear state and promoting a first-order switch under field sweep. ${\mathrm{KCoPO}}_4·{\mathrm{H}}_2\mathrm{O}$ thus provides a clean insulating platform and a microscopic standard for identifying quantum first-order transitions in frustrated magnets.

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• Antiferromagnetism
• First order phase transitions
• Frustrated magnetism
• Quantum phase transitions
• 2-dimensional systems
• Magnetization measurements
• Muon spin relaxation & rotation
• Neutron diffraction
• Specific heat measurements