Highly productive kelps release abundant particulate organic matter into the nearshore environment due to their constant fragmentation and erosion by ocean waves. The contribution of kelp detritus to coastal planktonic food webs has not previously been examined. Here, we demonstrate that detritus derived from a dominant kelp in the Northeast Pacific, Nereocystis luetkeana, provides high-quality food for planktonic sea urchin larvae. Our findings challenge the paradigm that phytoplankton are the main diet for zooplankton in nearshore regions, with implications for modeling of ocean production. Furthermore, at the benthic adult stage, sea urchins can destructively graze kelps causing the kelp ecosystem to collapse; hence, our results have implications for understanding feedback mechanisms that may determine the resilience of kelp ecosystems.