A manipulative field experiment to test for trophic cascading effects of predatory fish on detritus processing by benthic invertebrates was performed in stream channels running through a wetland forest in northern Japan. To control for fish effects on benthic invertebrates, two simple treatments (fish‐present and fish‐absent) were established for 4 weeks, with two common predatory fish, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin (Cottus nozawae), being introduced into and excluded from stream cages. At the end of experiment, the biomass of the dominant detritivore, an amphipod (Jesogammarus jezoensis), was significantly less in the fish‐present treatment (0.56 g m–2 in dry mass on average) than that in the fish‐absent treatment (1.32 g m–2), there being no significant treatment effect evident for the second‐dominant detritivore, coleopteran larvae (Optioservus kubotai). The loss of oak leaves (Quercus crispla) from litter bags in the fish‐present treatment (0.31 g week−1 in dry mass on average) was significantly less than in the fish‐absent treatment (0.54 g week−1). Predator‐induced lower biomass and likely lowered foraging activities of the J. jezoensis were responsible for the suppression of litter processing efficiency. In contrast, the standing crop of fine particulate organic matter did not differ significantly between the treatments. The experimental results revealed that the predatory fish had an indirect but significant effect on leaf litter processing in the stream.