Modifications of genes that encode proteins found exclusively in the tectorial membrane (TM) alter mechanical properties and produce a wide range of hearing deficits. However, the changes in TM physical properties responsible for these deficits remain unclear. In particular, the cochlear tuning of Tectb-/- mice is significantly sharper than that of TectaY1870C/+ mice, even though the stiffnesses of TectaY1870C/+ and Tectb-/- TMs are similarly reduced relative to wild-type TMs. Here we show that differences in TM wave properties that are governed by shear viscosity account for these differences in tuning. The shear viscosity of the TM results from the interaction of interstitial fluid with the porous structure of the TM's macromolecular matrix. In basal regions of the cochlea, nanoscale pores of TectaY1870C/+ TMs are significantly larger than those of Tectb-/- TMs. The larger pores in TectaY1870C/+ TMs gives rise to lower shear viscosity (by ∼70%), which in turn, reduces wave speed and increases wave decay constants relative to Tectb-/- TM wave properties. These results demonstrate the importance of TM porosity in cochlear tuning and that TM porosity, not stiffness, underlies the striking differences in hearing between TectaY1870C/+ and Tectb-/- mice.