Composite magnetoelectric compounds that combine ferroelectricity/piezoelectricity and ferromagnetism/magnetostriction are investigated intensively for room-temperature applications. Here, we studied bulk composites of a magnetostrictive constituent, ferromagnetic Fe 3 O 4 nanoparticles, homogeneously embedded in a ferroelectric/piezoelectric matrix, Pb(Zr 0.52 Ti 0.48 )O 3 (PZT). Specifically, we focused on PZT-5%Fe 3 O 4 samples which are strongly insulating and thus sustain a relatively high out-of-plane external electric field, E ex,z . The in-plane strain-electric field curve (S(E ex,z )) was carefully recorded upon successive application and removal of an out-of-plane external magnetic field, H ex,z . The obtained S(E ex,z ) data exhibited two main features. First, the respective in-plane piezoelectric coefficients, d(E ex,z ) = 200–250 pm/V, show a dramatic decrease, 50–60%, upon application of a relatively low H ex,z = 1 kOe. Second, the process is completely reversible since the initial value of d(E ex,z ) is recovered upon removal of H ex,z . Polarization data, P(E ex,z ), evidenced that the Fe 3 O 4 nanoparticles introduced static structural disorder that made PZT harder. Taken together, these results prove that the Fe 3 O 4 nanoparticles, except for static structural disorder, introduce reconfigurable magnetic disorder that modifies the in-plane S(E ex,z ) curve and the accompanying d(E ex,z ) of PZT when an external magnetic field is applied at will. The room-temperature feasibility of these findings renders the PZT-x%Fe 3 O 4 system a solid basis for the development of magnetic-field-controlled PE devices.