High-precision analysis of the <sup>17</sup>O / <sup>16</sup>O isotope ratio in water and water vapor is of interest in hydrological, paleoclimate, and atmospheric science applications. Of specific interest is the parameter <sup>17</sup>O excess (Δ<sup>17</sup>O), a measure of the deviation from a~linear relationship between <sup>17</sup>O / <sup>16</sup>O and <sup>18</sup>O / <sup>16</sup>O ratios. Conventional analyses of Δ<sup>17</sup>O of water are obtained by fluorination of H<sub>2</sub>O to O<sub>2</sub> that is analyzed by dual-inlet isotope ratio mass spectrometry (IRMS). We describe a new laser spectroscopy instrument for high-precision Δ<sup>17</sup>O measurements. The new instrument uses cavity ring-down spectroscopy (CRDS) with laser-current-tuned cavity resonance to achieve reduced measurement drift compared with previous-generation instruments. Liquid water and water-vapor samples can be analyzed with a better than 8 per meg precision for Δ<sup>17</sup>O using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy of Δ<sup>17</sup>O are competitive with conventional IRMS methods. The new instrument also achieves simultaneous analysis of δ<sup>18</sup>O, Δ<sup>17</sup>O and δ<i>D</i> with precision of < 0.03‰, < 0.02 and < 0.2‰, respectively, based on repeated calibrated measurements.