Understanding thermal transport in two-dimensional (2D) materials at cryogenic temperatures is essential for emerging low-temperature nanoelectronics, yet remains largely unexplored because of the lack of a predictive theoretical framework. Here, we develop a rigorous, accelerated, mode-resolved first-principles approach that enables the quantitative prediction of thermal conductivity in 2D materials down to 5 K. Using monolayer ${\mathrm{MoSe}}_2$ ribbons as a model system, we explicitly account for anharmonic, isotope, and boundary scattering, as well as finite-size effect, going beyond the conventional Matthiessen approximation. We reveal qualitatively distinct transport physics arising from reduced dimensionality. Unlike three-dimensional (3D) systems, where boundary-limited transport approaches the conventional Casimir regime with a $T^3$ dependence, 2D ${\mathrm{MoSe}}_2$ ribbons exhibit no well-defined Casimir regime; instead, their thermal conductivity follows geometry-sensitive power-law scaling ranging from $T^{1.9}$ to $T^{0.7}$. In addition, strain effects exhibit strong temperature- and size-dependent behavior. Strain effects are modest in nanoscale ribbons (∼100 nm) in which harmonic properties dominate, but enhance out-of-plane flexural branch phonon scattering in larger samples (∼1 mm), reducing conductivity by up to 50% at low temperatures. These results establish a quantitative framework for cryogenic heat transport in 2D materials with direct implications for the design of cryogenic 2D devices.

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• Anharmonic lattice dynamics
• Ballistic transport
• Thermal conductivity
• Thermal transport
• 2-dimensional systems
• Transition metal dichalcogenides
• Boltzmann theory
• Density functional theory