<p>Thermocapillary convection driven by surface-tension gradients governs interfacial transport phenomena across a wide range of length scales, from thin liquid films to molten material processing. In crystal growth technologies, high-purity bulk single crystals are typically fabricated using the Czochralski and floating-zone methods, where hydrothermal waves (HTWs) arising from thermocapillary instabilities can induce defects. The development of a digital twin for HTW dynamics has the potential to enable flow control and optimize crystal growth. However, such a framework requires three-dimensional (3D) velocity information, while the full 3D velocity structure of HTWs has not yet been experimentally resolved. In this study, we measure the 3D velocity field of a fully developed HTW in a thin liquid film using tomographic stereo particle image velocimetry (TSPIV), combined with simultaneous infrared (IR) thermography of the free-surface temperature. Experiments are conducted in a rectangular container filled with silicone oil (Prandtl number 16.1). Characteristic HTW features, including propagation angle, frequency, phase velocity, and spatial wavenumber, are quantified and show good agreement with previous studies. Importantly, the volumetric measurements reveal three-dimensional roll organization and its direct correspondence with surface temperature patterns, which cannot be inferred from previous two-dimensional or pointwise measurements. In particular, source and suction flow regions near the free surface are identified through coupled velocity–temperature analysis. These results provide new physical insight into the three-dimensional structure and thermal–flow coupling of HTWs and support the use of IR-based temperature data for constructing digital twins of thermocapillary convection.</p>