The rotational spectrum of N-tert-butylacetamide, CH3(C=O)(NH)C(CH3)(3), was measured in the frequency range from 2 to 26.5 GHz using a molecular beam Fourier transform microwave spectrometer. Only one, conformer with trans configuration and C-5 symmetry was observed. Torsional splittings up to 4.3 GHz occurred in the spectrum due to the internal rotation of the acetyl methyl group CH3(C=O) with a barrier height of approximately 65 cm(-1). Hyperfine structures arise from the quadrupole coupling of the N-14 nucleus appeared for all rotation torsional transitions. The data set was reproduced with the programs XIAM and BELGI-C-1-hyperfine, an extended version of the BELGI-C-1 code that includes the effect of the 14N quadrupole coupling, to root-mean-square deviations of 16.9 and 3.0 kHz, respectively. Quantum chemical calculations were performed to complement the experimental results. The BELGI-C-1-hyperfine code was also used to refit the recently published microwave data of N-ethylacetamide to measurement accuracy.