The frequencies and line strengths of the ^2^{Pi}3/2 J=5/2-3/2 rotational transition of ^17^OH have been calculated from an analysis of its far-infrared laser magnetic resonance spectrum. These results have been used to make the first detection of a pure rotational transition of ^17^OH in the ISM. Two resolved components of this transition appear in absorption towards the giant molecular cloud Sagittarius B2, which was observed at a spectral resolution of 33km/s with the Fabry-Perot mode of the ISO Long Wavelength Spectrometer. The corresponding transition of ^18^OH was also observed and its line shape was modelled using HI measurements. The ^18^O/^17^O ratio of 3.5 was then used to compare this with the observed ^17^OH line shape.