Ultra-high molecular weight polyethylene (UHMW-PE) fibers exhibit significantly higher elastic modulus and axial thermal conductivity (more than two orders of magnitude) than conventional bulk polymers. Such fibers are currently used in various applications owing to their high strength, but their promise for use as a flexible/wearable heat spreading materials in thermal management applications has not been extensively explored. While the thermal properties of individual fibers have been measured in the literature, it is crucial to characterize the thermal conductivity of fiber bundles (yarns) from which wearable fabrics and other materials would be constructed. This work presents a steady state infrared thermography-based method for thermal conductivity measurement of yarns. Current flowing through a calibrated heater wire generates heat that conducts through the yarn sample, which is placed perpendicularly to the heater wire. Steady state temperature maps are obtained with and without the yarn sample in contact with the heater wire to extract the effective thermal conductivity of the yarn using an associated one-dimensional heat transfer model. The technique does not require estimation or knowledge of the thermal contact resistance between the heating wire and yarn sample. The method is validated by measuring the thermal conductivity of a known reference material. The effective thermal conductivity of an UHMW-PE yarn is measured to be approximately 16 Wm^‑1K^‑1, slightly lower than that of individual fibers due to the non-continuous nature of the fibers within the yarn. The developed measurement technique is broadly useful for thermal characterization of microscale strands or other rod-like structures.