This dataset is intended to support the publication "Experimental study of the impact of blade-tip mounted rotors on the X-Rotor vertical-axis wind turbine" by Bensason, Sciacchitano, and Ferreira, published in the Journal of Physics: Conference series under DOI 10.1088/1742-6596/2767/7/072016. The dataset includes wake data obtained using Particle Image Velocimetry (PIV) in the open-jet wind tunnel facility of the TU Delft (OJF) for a scaled X-Rotor vertical-axis wind turbine (VAWT) with a diameter of 1.5m, coined the primary rotor. The measurements focus on the bottom blade tips, which have blade-tip mounted rotors (approximately 10cm in diameter), coined secondary rotors. This study aims to evaluate the temporal and spatial evolution of the secondary rotor wake during the upwind passage of the primary rotor. Additionally, it provides an experimental demonstration of the working principle of the X-Rotor concept, coined the “aerodynamic gearbox”.