<p>The motion of a quantum system under an external force often challenges classical intuition. A notable example is the dynamics of a single particle in a periodic potential, which undergoes Bloch oscillations under the action of a constant force. Similar oscillations can also occur in one-dimensional quantum fluids without a lattice. The generalization of Bloch oscillations to a weakly bounded ensemble of interacting particles has so far been limited to the experimental study of the two-particle case, where the observed period is halved compared to the single-particle case. In this work, we observe the oscillations of the position of a mesoscopic solitonic wave packet—consisting of approximately 1,000 atoms—in a one-dimensional Bose gas subjected to a constant uniform force and in the absence of a lattice potential. The oscillation period scales inversely with the number of atoms, revealing its collective nature. We demonstrate the role of the phase coherence of the quantum bath in which the wave packet moves and investigate the underlying topology of the associated superfluid currents. Our measurements highlight the periodicity of the dispersion relation of collective excitations in one-dimensional quantum systems.</p>