Measuring the thermal conductivity of high-temperature liquids is essential for advanced nuclear energy systems, where accurate thermophysical property data support reactor design, safety analysis, and model validation. In this study, we develop and demonstrate a fiber-optic frequency-domain thermoreflectance (fiber-FDTR) technique tailored for in situ measurements in high-temperature liquid environments. We report relative thermal conductivity measurements for liquid gallium up to 350 °C, as well as absolute values for Ga at 45.5 °C, Bi at 280 °C, and Hg at 25 °C. Absolute conductivities were obtained by fitting experimental data to a heat-transfer model, yielding uncertainties of ∼20%, dominated by the precision of the mode-field diameter. Steady-state thermoreflectance measurements provided relative thermal conductivity curves for gallium with excellent repeatability and low uncertainty (3.8%). These results establish fiber-FDTR as a flexible, high-throughput approach for characterizing thermophysical properties of high-temperature liquids, particularly in geometries where optical access through windows is impractical. The technique shows strong potential for extension to molten salts and other harsh liquid systems.