Different amounts (5-30 wt% with respect to the polyurethane content in the adhesive) of a new filler, a partially defibrillated natural hydrated magnesium silicate (sepiolite), were added to solvent-based polyurethane (PU) adhesive formulations. The rheological, mechanical, surface, and adhesion properties of the PU adhesives obtained were measured. Increase in the amount of sepiolite added to PU adhesives led to an increase in viscosity and imparted thixotropy and pseudoplasticity to the adhesive solution. However, the addition of sepiolite produced an increase in storage and loss moduli and a decrease in the mechanical properties but did not affect the surface properties of the PU adhesive films. On the other hand, the immediate (measured 30 s after joint formation) T-peel strength of roughened or (roughened + chlorinated with 1 wt% trichloroisocyanuric acid solutions in 2-butanone) styrene-butadiene rubber/PU adhesive joints was greatly improved if the adhesive was filled with up to 10 wt% sepiolite. The T-peel strength measured 72 h after bond formation was similar for the joints prepared with PU adhesives without and with up to 10 wt% sepiolite. The joint strength decreased if the amount of sepiolite in the PU adhesive was 20-30 wt%, due to the poor mechanical properties and too high moduli of the adhesives. Some interactions between the sepiolite, the polyurethane, and/or the solvent seemed to be responsible for the improved properties of filled PU adhesives. These interactions were responsible for the increased storage and loss moduli and the displacement of Tg Of the PU adhesive films to higher temperature when it contained sepiolite. Furthermore, the addition of a high amount of sepiolite changed the rheological behaviour of the PU to a solid, giving rigidity to the structure, which is responsible for the reduced adhesion in roughened rubber joints produced with polyurethanes containing 20-30 wt% sepiolite.