Octahedral tilting is a fundamental structural distortion in perovskites, governing key phenomena such as lattice stabilizing, soft phonon dynamics, group-theoretical analysis, phase transitions, ferroelectricity, and even for tunable electronic band gap. However, its influence on lattice thermal conductivity $\left({\kappa }_L\right)$ remains poorly understood. In the archetypal perovskite ${\mathrm{SrTiO}}_3$, tilting in the low-temperature tetragonal phase is known to enhance ${\kappa }_L$ by suppressing specific phonon scattering channels around $200{\mathrm{cm}}^{-1}$. Here, we investigate the thermal transport in strontium stannate $\left({\mathrm{SrSnO}}_3\right)$, another perovskite oxide that undergoes temperature-driven phase transitions, and reveal a completely opposite effect. Through a systematic study across its orthorhombic, tetragonal, and cubic phases, we demonstrate that octahedral tilting in the tetragonal phase of ${\mathrm{SrSnO}}_3$ anomalously triggers acoustic phonon softening. This softening manifests as reduced frequencies and group velocities in low-frequency (<3 THz) acoustic modes, creating a large decrease for heat transport, particularly along the $c$ axis. Consequently, ${\kappa }_L$ is significantly suppressed, decreasing from $7.48\mathrm{W}{\mathrm{m}}^{-1}{\text{K}}^{-1}$ to $6.06\mathrm{W}{\mathrm{m}}^{-1}{\text{K}}^{-1}$ as the tilting angle increases by a mere $1^{\circ }$. These findings identify tilting-induced acoustic softening as a pivotal mechanism for limiting and controlling anisotropic thermal transport in ${\mathrm{SrSnO}}_3$, presenting a stark contrast to the established behavior in ${\mathrm{SrTiO}}_3$.

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• Lattice thermal conductivity
• Phonons
• Perovskites
• Density functional calculations