The recent volcanic unrest on Iceland's Reykjanes Peninsula was an excellent opportunity to better understand volcanic processes and develop hazard mitigation strategies. The eruption was studied using various direct and remote-sensing techniques. Here, we present an innovative UAV-based TomoSAR approach application, combined with photogrammetry, to explore the external and internal structures of an active volcanic vent within the Sundhnúkur crater row, where nine eruptions have occurred since December 2023. The surveys were conducted on 20 May 2024 (12 days after the end of the March–May eruption) and on 1 August 2024 (40 days after the May–June eruption). For optical data collection, we used a DJI Mavic 3T quadcopter, equipped with an RGB camera and an infrared sensor. The radar data were acquired using a UAV-based interferometric SAR system, Explorer RD350, which is capable of collecting P-band data in helical-trajectory mode. The optical data were processed using the standard photogrammetric workflow, and the SAR data were processed using the Refractive Back Projection algorithm, which enabled the extraction of amplitude images as slices at given depths with a ground penetration of up to 20 m. Our results show that the higher-intensity areas in the subsurface images correspond to the vent's crater center, while the lower-intensity areas correspond to the slopes of its cinder cone, composed of loose volcanic material. We assume that the higher-intensity areas in the amplitude images represent structures of denser material at depth, e.g., a lava conduit within the volcanic cone.