The molecular geometry of yttrium trichloride has been determined by high-temperature gas-phase electron diffraction. The vapor phase consisted of about 87% monomeric and 13% dimeric species. High-level quantum chemical calculations have also been carried out for both the monomer and dimer of yttrium trichloride, and their geometries, harmonic force fields, and vibrational frequencies have been determined. The monomer YCl3 molecule was found to be planar (D-3h symmetry) both by experiment and by computation. The bond length Of YCl3 from electron diffraction is 2.450(7) (r(g)) or 2.422(12) (r(e)) Angstrom. It proved remarkably difficult to obtain a converged theoretical prediction for the bond length; large polarization bases are needed, and the published bases accompanying the pseudopotentials used appear to be overcontracted. The SCF method predicts bonds that are too long by some 0.043 Angstrom, whereas the B3LYP method overestimates by about 0.03 Angstrom. The B3PW91 prediction is almost within the experimental uncertainty for r(e). Among the traditional correlated methods, the MP2 distance with an infinitely large basis is probably indistinguishable from the experimental value, given the combined uncertainties, whereas the estimated CCSD(T) result of 2.423(10) Angstrom is astonishingly close to the experimental result. The out-of-plane bending motion for YCl3 is noticeably anharmonic, with the result that straightforward quantum predictions of its frequency are lower than the value observed in the gas phase at high temperature.