$\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$, a representative member of the quaternary diamondlike compound family, exhibits intrinsic Dirac states and promising thermoelectric performance due to its intrinsically low thermal conductivity. However, the physical mechanisms underlying its ultralow κ remains poorly understood. Here, we successfully control the concentration of $\mathrm{C}{\mathrm{u}}_{\mathrm{Hg}}$ antisite defects (ASDs) in $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ single crystals by tuning the growth conditions. The two types of $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ crystal samples are found to have very different $\mathrm{C}{\mathrm{u}}_{\mathrm{Hg}}$ ASDs concentrations using the single-crystal x-ray structure refinements. Thermal conductivity measurements at room temperature show that the sample with a high concentration of ASDs exhibits an ultralow lattice thermal conductivity of only $0.44\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, nearly half of that of the low-defect sample. First-principles calculations for the two $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ crystals with and without ASDs are in good agreement with the experimental data, confirming the pivotal role of $\mathrm{C}{\mathrm{u}}_{\mathrm{Hg}}$ ASDs in suppressing lattice thermal conductivity. These findings provide direct insight into the structure–property relationship in $\mathrm{C}{\mathrm{u}}_2\mathrm{HgSnS}{\mathrm{e}}_4$ and demonstrate that defect engineering is an effective strategy to tailor thermal transport in quaternary diamondlike semiconductors, with implications for enhanced thermoelectric and photovoltaic performance.

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• Defects
• Lattice thermal conductivity
• Single crystal materials
• Thermoelectrics
• Crystal growth
• Density functional calculations
• Thermoreflectance
• X-ray diffraction