With the rapid development of integrated electro-optic and nonlinear optical devices based on lithium niobate (${\mathrm{LiNbO}}_3$, LN), thermal management is becoming a critical area of focus. However, experimental measurement of thermal transport in stoichiometric ${\mathrm{LiNbO}}_3$ (sLN) remains scarce, and the intrinsic microscopic mechanisms remain to be established. Here, we combine the laser pump-probe technique of frequency-domain thermoreflectance (FDTR) with state-of-the-art machine-learned atomistic simulations to comprehensively investigate thermal transport in sLN. The measured and simulated room-temperature thermal conductivity ($\kappa$) values of sLN agree well, which are orders of magnitude lower than those of many classic and emerging semiconductors such as silicon. Furthermore, the temperature-dependent $\kappa$ exhibits $T^{-\alpha }$ scaling with $\alpha$ near unity, suggesting that thermal transport is dominated by intrinsic phonon-phonon scattering. By comparing sLN with cubic boron arsenide (cBAs) which serves as an ultrahigh-$\kappa$ benchmark, we reveal that harmonic properties are not responsible for the low $\kappa$ of sLN, which feature phonon heat capacity and group velocities that are either higher than or comparable with those in cBAs. Instead, the low $\kappa$ originates from substantially stronger anharmonicity and larger scattering phase space. These two factors collectively suppress phonon lifetimes by 1–2 orders of magnitude, leading to a maximum phonon mean free path of approximately 140 nm. As a result, notable size effects emerge in thin-film sLN below 1 µm, with $\kappa$ dropping to half the bulk value at 10 nm. Altogether, our findings establish a fundamental understanding of thermal transport in sLN and provide atomistic insights for thermal management in advanced lithium niobate technologies.

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• Anharmonic lattice dynamics
• Thermal conductivity
• Machine learning
• Molecular dynamics
• Pump-probe spectroscopy