The HCN trimer aminomalononitrile (H2NCH(CN)(2), AMN) is considered as a key compound in prebiotic chemistry and a potential candidate for detection in the interstellar medium. In this view, we studied the rotational spectrum of AMN in the 120-245 GHz frequency range. The spectroscopic work was augmented by high-level ab initio calculations. The calculations showed that between two existing rotamers, symmetric and asymmetric, the most stable is the asymmetric conformation, and it is the only conformation observed in the recorded spectra. The symmetric conformation is 6.7 kJ/mol higher in energy and thus has a very low Boltzmann factor. The analysis of the rotational spectra of the A conformation has shown that the observed lines exhibit a doublet or quartet structure owing to two large-amplitude motions, C-N torsion and amino group inversion. To study the large-amplitude motions in detail, we calculated a two-dimensional potential energy surface and determined the barrier heights for the torsion and inversion, Vt = 12.5 kJ/mol and Vi = 19.1 kJ/mol. About 2500 assigned rotational transitions in the ground vibrational state were fitted within experimental accuracy using the reduced axes system Hamiltonian. The set of obtained spectroscopic parameters allows accurate calculation of transition frequencies and intensities for an astrophysical search of AMN.