Spider silk exhibits a combination of outstanding tensile strength and extensibility unique among all synthetic and biogenic polymer fibers. It has thus generated great interest to understand protein-based high-toughness materials and inspired the design of similar synthetic materials. The unrivaled properties of silk fibers have been recognized to be intimately related to their hierarchical structure. However, in the absence of unambiguous experimental evidence, competing and incompatible structural models of natural silk fibers have been proposed, some of them including various types of fibrillar components. Here we show that the fibers of the recluse (Loxosceles) spider exhibit the typical tensile properties of a very good spider silk and are entirely composed of 20 nm diameter protein fibrils that are more than 1 μm long. Based on these findings, we developed the most detailed structural model for any silk directly supported by experimental evidence. Our work suggests that all the key properties of a spider silk are implemented within a single nanofibril, and we have isolated and imaged such a nanofibril from a native spider silk fiber. The nanofibril breaking force was estimated to be ≈120 nN. Our work underlines the importance of nanofibrils and furthers the understanding of the structure-property relationships of silk, with wide-ranging implications for silk research and the design of silk-inspired high-performance materials.