Low-dimensional crystalline materials exhibiting large atomic fluctuations enable low thermal conductivity for many applications, yet the critical interplay between thermal anisotropy and lattice anharmonicity remains elusive. Here, we combine first-principles calculations with unified thermal transport theory to uncover the unconventional thermal transport anisotropy correlated with the striking quartic anharmonicity in a quasi-two-dimensional compound $\mathrm{C}{\mathrm{u}}_2{\mathrm{P}}_2{\mathrm{O}}_7$. We find that the unique quasiplanar atomic network and hierarchical bonding create a permissive environment for intense oscillations of interlayered O atoms, triggering giant quartic anharmonicity. This, combined with small phonon interbranch spacing, renders wavelike coherences the dominant mechanism for heat transport, resulting in a nonmonotonic temperature dependence of lattice thermal conductivity and a reversal of its anisotropy between in-plane and out-of-plane directions. Our findings yield fundamental insights into the coherence-driven thermal transport anomaly in highly anharmonic low-dimensional crystals, which would guide the rational design of advanced thermal materials.

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• Anharmonic lattice dynamics
• Lattice thermal conductivity
• Layered semiconductors
• Ab initio molecular dynamics
• Boltzmann theory