We measured seismic attenuation in the frequency range from 1 to 100 Hz and transient fluid pressure in a partially saturated rock sample. The pressure sensors were located 4.2 cm apart and, therefore, provided information on pressure gradients in the mesoscopic scale. The sample was of Berea sandstone and was saturated with 97% water (3% air). The measurements were performed at room pressure and temperature. The laboratory results suggested that that wave-induced fluid flow in the mesoscopic scale is dominant in partially saturated samples. To verify that, we performed numerical modeling to compute attenuation and transient fluid pressure on a three-dimensional poroelastic model representing the rock sample. The finite-element method was used to solve Biot's equations of consolidation by performing a quasi-static creep test. Wave-induced fluid flow in the mesoscopic scale is the only attenuation mechanism accounted for in the numerical solution. The numerical results reproduced the laboratory data for transient fluid pressure. Likewise, the numerically calculated attenuation, superposed to the frequency-independent attenuation measured in the dry sample, reproduced the attenuation measured in the laboratory in the partially saturated sample. These results show that wave-induced fluid flow in the mesoscopic scale is the dominant mechanism for frequency-dependent seismic attenuation in fluid-saturated Berea sandstone.