We have observed a sharp absorption edge at 2.7 microns in the near-IR spectrum of SN 1987A on day 53 after the progenitor star's core collapsed. We attribute this feature to structural OH in silicate minerals in circumstellar dust ejected from the progenitor star during its red supergiant phase. We obtain a mass column density of 4.3 × 10-4 g cm-2 (±7%) for the absorbing material in the line of sight toward SN 1987A. The radii of the dust particles must be smaller than 0.04 microns to be consistent with the low value of interstellar extinction toward SN 1987A. A region of broad absorption extends from the long-wavelength side of the absorption edge at 2.7 microns which suggests that interlayer H2O is also present in these circumstellar silicates. Our estimates of the vapor phase hydration of silicate grains in the atmospheres of red supergiants indicate that the reactions are not necessarily kinetically inhibited. No silicate absorption feature has been observed at 10 microns in spectra of SN 1987A at any time. We suggest that it may have been suppressed by an IR echo emitted from circumstellar dust around SN 1987A.