A molecular complex of C2H2 and AuI has been generated and isolated in the gas phase through laser ablation of a gold surface in the presence of an expanding sample containing small percentages of C2H2 and CF3I in a buffer gas of argon. Rotational, B0; centrifugal distortion, DJ and DJK; and nuclear quadrupole coupling constants, χaa(Au), χbb(Au)-χcc(Au), χaa(I) and χbb(I)-χcc(I); are measured for three isotopologues of C2H2···AuI through broadband rotational spectroscopy. The complex is C2v and T-shaped with C2H2 coordinating to the gold atom via donation of electrons from the pi-orbitals of ethyne. On formation of the complex, the C-C bond of ethyne extends by 0.032(4) Å relative to r(C-C) in isolated ethyne when the respective r0 geometries are compared. The geometry of ethyne distorts such that (*-C-H) (where * indicates the midpoint of the C-C bond) is 194.7(12) degrees in the r0 geometry of C2H2···AuI. Ab initio calculations at the CCSD(T)(F12*)(AVTZ) level are consistent with the experimentally-determined geometry and further allow calculation of the dissociation energy as 136 kJ mol-1. The χaa(Au) and χaa(I) nuclear quadrupole coupling constants of AuI and also the Au-I bond length change significantly on formation of the complex consistent with the strong interaction calculated to occur between C2H2 and AuI.