The weakly bound cyclic trimer (H2O)(2)HBr was observed in supersonic expansion and its rotational spectrum was measured in the region 3.6-17.7 GHz by cavity Fourier-transform microwave spectroscopy. Rotational, centrifugal distortion constants, and inertial and principal hyperfine splitting constants were determined for seven isotopomers of (H2O)(2)HBr. In addition to the large bromine hyperfine splitting each rotational transition exhibits a fine vibrational splitting into four components, at relative intensities consistent with expectations from the G(8) group classification of the vibration-rotation-tunneling motions in the trimer. The associated four low-lying states are either very close together or well below the inversion barriers, since the differences between their rotational constants are all below 0.02%. The experimental moments of inertia were used to determine r(s), r(0), r(z), and r(m)((1L)) geometries, in all of which the heavy atom distances are considerably shorter than similar distances in H2O-HBr and (H2O)(2). An improved analysis of the measured electric dipole moment of (H2O)(2)HBr is reported, and all experimental results are confronted with predictions from ab initio calculations. (C) 2003 American Institute of Physics.