Abstract The rotational spectra of cyanoacetylene dimer, H C C C N⋯H C C C N, were recorded using Balle–Flygare type Fourier transform microwave (FTMW) spectrometers. The low J transitions were measured down to 1.3 GHz at very high resolution, FWHM ∼ 1 kHz. The spectral hyperfine structure due to the 14N nuclear quadrupole coupling interactions is well-resolved below 4 GHz using a low frequency spectrometer at the University of Arizona. The experimental spectroscopic constants were fitted as: B0 = 339.2923310(79) MHz, DJ = 32.152(82) Hz, H = −0.00147(20) Hz, eqQ(14N1) = −3.9902(14) MHz, and eqQ(14N2) = −4.1712(13) MHz. The vibrationally averaged dimer configuration is H C C C N⋯H C C C N. Using a simple linear model, the vibrational ground state and the equilibrium hydrogen bond lengths are determined to be: r0(N⋯H) = 2.2489(3) A and re(N⋯H) = 2.2315 A. The equilibrium center-of-mass distance between the two HCCCN subunits is rcom = 7.0366 A. Using the rigid precession model, the vibrational ground state center-of-mass distance and the pivot angles which HCCCN subunits make with the a-axis of H C C C N⋯H C C C N are rc.m. = 7.0603 A, θ1 = 13.0°, and θ2 = 8.7°, respectively. The calculated hydrogen bond energy of H C C C N⋯H C C C N is 1466 cm−1 using the MP2/aug-cc-PVTZ method in present work.