Clathrate hydrates are crystalline compounds in which guest molecules are encaged within an ice-like lattice. They occur naturally and possess properties of significant interest for energy and storage applications. Here, we report the thermal conductivity κ of structure I CO2 clathrate hydrate across a broad temperature range (90–265 K) and at pressures up to 1.2 GPa. Similar to structure II clathrate hydrates, κ decreases with decreasing temperature, displaying almost identical temperature dependence. However, the absolute values are 10–30% lower. Notably, κ of CO2 clathrate hydrate is among the lowest observed for structure I clathrate hydrates, with κ = (426 ± 8) mW m–1 K–1 under stable conditions at 270 K and 1 MPa. Furthermore, the isothermal dependencies of κ on density ρ and pressure p─parameters crucial for thermal modeling at elevated pressures─are relatively weak, with (d ln κ/d ln ρ) = 1.2 ± 0.2 and (d ln κ/dp) = (12 ± 1) % GPa–1 . The measurements show significantly lower κ values and a different temperature dependence compared with previously reported simulation results. Nevertheless, the experimental data confirm the simulation prediction that κ for CO2 clathrate hydrate is significantly lower than for other structure I clathrates. Our findings further indicate that κ in both structures I and II clathrate hydrates tends to decrease with increasing van der Waals radius of the guest molecules, as reviewed here. This trend may arise from enhanced distortion and anharmonicity within the ice framework. We tentatively propose that the pronounced anharmonicity of the clathrate hydrate lattice leads to frequent phonon–phonon scattering, effectively suppressing phonon-mediated heat transport and resulting in predominantly diffusive thermal conduction.