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An ab initio characterization of the impact of tert‐Butyl isocyanate (C5H9NO${\rm C}_5{\rm H}_9{\rm NO}$) functionalization on TaS2${\rm TaS}_2$ lattice thermal conductivity is presented. Such a system has been previously synthetized and experimentally charachterized, showing that the incorporation of covalently bonded C5H9NO${\rm C}_5{\rm H}_9{\rm NO}$ on bulk TaS2${\rm TaS}_2$ leads to a dramatic in‐plane lattice thermal conductivity decrease by more than two orders of magnitudes. To elucidate these experimental findings, detailed calculations of the phonon dispersion relations and scattering rates in TaS2${\rm TaS}_2$ are performed. The analysis is addressed to discern the impact of inter‐layer covalently bonded C5H9NO${\rm C}_5{\rm H}_9{\rm NO}$ molecules on these phonon properties, providing insights into the underlying mechanisms of the observed thermal conductivity decrease. Present calculations provide evidence that the observed lattice thermal conductivity reduction is attributed to two effects: i)$i)$ the increase of inter‐layer separation and ii)$ii)$ the presence of low‐frequency molecular optical modes. The first inhibits specific Van der Waals quasi‐acoustic inter‐layer vibrational modes contributing as much as ≈55%$\approx \!\! 55\%$ in bulk TaS2${\rm TaS}_2$. The second effect dramatically decreases the phonon group velocities as a consequence of phonon‐crossing phenomena among low‐frequency molecular modes and TaS2${\rm TaS}_2$ acoustic modes, eventually leading to a strong reduction of all phonon lifetimes. [ABSTRACT FROM AUTHOR]