Determination of the density and ionization state of diffuse primordial gas in the intergalactic medium (IGM) is an important step towards understanding the nature of dark matter and the formation of structure in the Universe. Absorption by neutral hydrogen atoms in the IGM should produce a large flux deficit in the spectra of high-redshift quasars at wavelengths below that of the H I Lyman-alpha line (the Gunn-Peterson effect(1)). But observations of quasars at low spectral resolution have revealed only relatively small deficits, and at higher spectral resolution these appear as numerous discrete absorption features (the Lyman-alpha forest). This implies that intergalactic hydrogen is both highly ionized and non-uniformly distributed(2). Similar absorption is expected hom intergalactic helium(3), although its detection is more challenging and evidence for its existence was obtained only recently(4,5). Here we report the observation of absorption from singly ionized helium (He II) in the spectrum of the quasar HS1700 + 64. We measure a mean flux deficit over the redshift range 2.2 < z < 2.6 that yields an effective optical depth of 1.00 (+/-0.07). Comparing this result with the H I absorption towards this quasar, we argue that most of the measured He II absorption occurs in the lowest-density regions of the IGM.