High-redshift Lyman-alpha (Ly alpha) blobs(1,2) are extended, luminous but rare structures that seem to be associated with the highest peaks in the matter density of the Universe(3-6). Their energy output and morphology are similar to those of powerful radio galaxies(7), but the source of the luminosity is unclear. Some blobs are associated with ultraviolet or infrared bright galaxies, suggesting an extreme starburst event or accretion onto a central black hole(8-10). Another possibility is gas that is shock-excited by supernovae(11,12). But not all blobs are associated with galaxies(13,14), and these ones may instead be heated by gas falling into a dark-matter halo(15-19). The polarization of the Lya emission can in principle distinguish between these options(20-22), but a previous attempt to detect this signature returned a null detection(23). Here we report observations of polarized Ly alpha from the blob LAB1 (ref. 2). Although the central region shows no measurable polarization, the polarized fraction (P) increases to similar to 20 per cent at a radius of 45 kiloparsecs, forming an almost complete polarized ring. The detection of polarized radiation is inconsistent with the in situ production of Ly alpha photons, and we conclude that they must have been produced in the galaxies hosted within the nebula, and re-scattered by neutral hydrogen.