[en] Microwave rotational spectra have been observed for the four ¹⁴N/¹⁵N isotopic species of an (HCN)₂--Ar trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. Thirteen J→J' asymmetric top transitions were measured in the 2.5 to 10 GHz region for the parent ¹⁴N,¹⁴N trimer and nine for each of the other three species. The ¹⁴N nuclear quadrupole hyperfine structure was analyzed for the transitions and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational and quartic centrifugal distortion constants. For the ¹⁴N/¹⁴N trimer these are (in MHz) for A'', B'', and C'': 2013.5993(10), 1759.2756(3), 932.3709(2); and for tau₁, tau₂, tau/sub a//sub a//sub a//sub a/, tau/sub b//sub b//sub b//sub b/, and tau/sub c//sub c//sub c//sub c/: -0.3017(1), -0.0660(1), -0.1700(9), -0.013 71(1), and -0.0088(1). The inertial defect is 3.7881 amu A². The zero-point vibrationally averaged geometry is planar and T shaped with the structure of the linear HCN dimer remarkably unperturbed by presence of the Ar atom. The substitution NxxxN distance in the cluster is 4.453 A compared with 4.461 A in the free dimer. The argon is attracted by the π electrons of the two --CequivalentN groups, being equidistant from them. The ArxxxN(1) and ArxxxN(2) distances are 3.671 and 4.316 A, respectively