Large-scale coupled cluster calculations have yielded an almost T-shaped equilibrium structure for the Ar-HCCCN van der Waals complex, with an equilibrium dissociation energy of D-e=221 cm(-1). The theoretical predictions are confirmed by pulsed-nozzle Fourier transform microwave spectroscopy in the frequency range from 3 to 26 GHz. Strong b-type and much weaker a-type transitions have been observed, and the N-14 nuclear quadrupole hyperfine components of the rotational transitions have been resolved. The spectral analysis has produced rotational, centrifugal distortion, and N-14 nuclear quadrupole coupling constants. Structural parameters describing a (vibrationally averaged) T-shaped complex geometry have been derived. (C) 2003 American Institute of Physics.