We present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the Japanese F-net array located in the Ryukyu arc. We obtained high-quality splitting measurements for 70 event-station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction. Splitting times for individual measurements ranged from 0.25 to 2 s; most values were between 0.75 and 1.25 s. Both the fast directions and the split times were similar to results for teleseismic S(K)KS and S wave splitting at the same stations, which suggests that the anisotropy is located in the mantle wedge above the slab. We considered several mantle deformation scenarios that would result in predominantly trench-parallel fast directions, and concluded that for the Ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with B-type olivine fabric. In this model, the flow direction in the wedge is perpendicular to the trench, but the fast axes of olivine crystals tend to align perpendicular to the flow direction, resulting in trench-parallel shear wave splitting.