Spider silk—an environmentally friendly protein-based material—is widely recognized for its extraordinary mechanical properties. Biomimetic spider silk-like fibers made from recombinant spider silk proteins (spidroins) currently falls short compared to natural silks in terms of mechanical performance. In this study, it is discovered that spiders use structural conversion of molecular enhancers—conserved globular 127-residue spacer domains—to make strong silk fibers. This domain lacks poly-Ala motifs but interestingly contains motifs that are similar to human amyloidogenic motifs, and that it self-assembles into amyloid-like fibrils through a non-nucleation-dependent pathway, likely to avoid the formation of cytotoxic intermediates. Incorporating this spacer domain into a recombinant chimeric spidroin facilitates self-assembly into silk-like fibers, increases fiber molecular homogeneity, and markedly enhances fiber mechanical strength. These findings highlight that spiders employ diverse strategies to produce silk with exceptional mechanical properties. The spacer domain offers a way to enhance the properties of recombinant spider silk-like fibers and other functional materials.