Reported here are the first spectroscopic observations of the nitrous oxide trimer. Two rotationally resolved vibrational bands have been observed using the optothermal detection method. The ground vibrational state rotational constants obtained from these spectra are A = 0.052606(40), B = 0.043571(15), and C = 0.028736(7) cm(-1). Ab initio calculations are also reported here, at the MP2/6 - 31 + G(2d, 2p) level, which yield A = 0.05492, B = 0.01517, and C = 0.03052 cm(-1). The structure of the nitrous oxide trimer can be thought of as a slightly distorted dimer, which has a slipped parallel geometry [Z. S. Huang and R. E. Miller, J. Chem, Phys. 89, 5408 (1988)], with a third monomer unit lying above this dimer, This structure is similar to the recently observed nonplanar isomer of (CO2)(3) [M. J. Weida and D. J. Nesbitt, J. Chem. Phys. 105, 10, 210 (1996)].