T-carbon, as one of new carbon allotropes that was first predicted and then successfully synthesized, has been attracting intensive research interest in recent years and has emerged potential applications in various areas. Here we use the frequency-domain thermoreflectance technique to measure for the first time the thermal conductivity (κ) of T-carbon, and report the power law behavior of κ(T) ∼ T−0.75 in T-carbon, with a value of 31.9 Wm−1K−1 at 300 K, which are nicely consistent with first-principles calculations (33.06 Wm−1K−1). Among all existing carbon crystals, we find that T-carbon has the lowest thermal conductivity, being nearly 50 times lower than that of cubic diamond, which is caused by the large scattering phase space and strong phonon anharmonicity of C–C bonds with sp3 hybridization in T-carbon. Our study reveals that T-carbon with particularly low thermal conductivity could have potential applications in energy converting devices, and the analyzed mechanism would deepen our understanding on the thermal transport and chemical bonding of carbon crystals. Display omitted