We present the discovery of an absorption-line redshift of z = 2.609 for GRB090426, establishing the first firm lower limit to a redshift for a gamma-ray burst (GRB) with an observed duration of <2s. With a rest-frame burst duration of T$_90z$ = 0.35s and a detailed examination of the peak energy of the event, we suggest that this is likely (at >90 per cent confidence) a member of the short/hard phenomenological class of GRBs. From analysis of the optical-afterglow spectrum we find that the burst originated along a very low HI column density sightline, with N$_HI$ < 3.2 × 10$^19$cm$^-2$. Our GRB090426 afterglow spectrum also appears to have weaker low- ionization absorption (SiII, CII) than åisebox-0.5ex 95 per cent of previous afterglow spectra. Finally, we also report the discovery of a blue, very luminous, star-forming putative host galaxy (i̊sebox-0.5ex 2L$_*$) at a small angular offset from the location of the optical afterglow. We consider the implications of this unique GRB in the context of burst duration classification and our understanding of GRB progenitor scenarios.