In this study, the flocculation and the subsequent decantation step of mixed suspensions of 10 nm-sized γ-Fe2O3 magnetic nanoparticles and 500 nm-sized beidellite clay platelets was investigated. This work may find application in the field of water treatment, specifically the flocculation processes with magnetically assisted sedimentation. After a short description of the preparation and characterization of the raw materials (nanoparticles and clays), the influence of several parameters (pH, concentrations of nanoparticles and clays etc.) on the amount of flocculated materials was examined, which gave information on the concentration ranges allowing a complete flocculation, together with a better understanding on the interactions between nanoparticles and clays responsible for flocculation. The optimal conditions for magnetically assisted settling were then determined by comparing for each sample sedimentation velocities under gravity and in the presence of a Nd-Fe-B magnet. Finally, the complex multiscale structure of the flocs in water was explored, through the measurement of several bulk properties (zeta-potential and volume measurements, laser granulometry), while the organization of the materials at a microscopic scale was investigated by cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS).