Quantum magnets offer a unique platform for exploring exotic quantum phases and quantum phase transitions through external magnetic fields. A prominent example is the field-induced Bose–Einstein condensation (BEC) of magnons near the saturation field. While this behavior has been observed in low-spin systems, its realization in high-spin, quasi-two-dimensional magnets—where multiple on-site excitations are possible—remains exceptionally rare. Here, we report thermodynamic and density functional theory results on single crystals of the honeycomb-lattice antiferromagnet ${\mathrm{K}}_4{\mathrm{MnMo}}_4{\mathrm{O}}_{15}$ with $S=\frac{5}{2}$. The system undergoes a field-induced transition to a fully polarized state at the critical field ${\mu }_0{H}_{\mathrm{s}}=6.4$ T. Our results reveal possible thermodynamic signatures of magnon BEC, $T_{\mathrm{N}}\sim {(H_{\mathrm{s}}-H)}^{2/d}$ ($d=3$), expanding the purview of BEC-driven quantum criticality to a high-spin, quasi-two-dimensional antiferromagnets with negligibly small anisotropy.

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• Magnetism
• Antiferromagnets
• Bose-Einstein condensates
• SQUID
• Crystal growth
• DFT+U
• Specific heat measurements
• X-ray diffraction