Chlorine nitrate is a molecule of both spectroscopic and atmospheric interest. It is a species whose spectrum in the infrared is marginally resolved in the Doppler limit, thereby limiting the spectroscopic study of its dense rovibrational structure. Analyses of its pure rotational spectrum in the millimeter and submillimeter (mm/submm) spectral regions of the ground, ν9, 2ν9, ν7, 3ν9, and ν7ν9 vibrational states have been previously reported. In this paper, we report the recording of a new ‘complete’ mm/submm spectrum from 78 to 378 GHz using phase-lock and FASSST (FAst Scan Submillimeter Spectroscopic Technique) systems. We also report the use of these new data for redetermination of spectroscopic constants for the ground state and for an analysis of the ν5/ν6ν9 interacting dyad, whose corresponding infrared spectrum lies near 560 cm−1. The simulation of the rotational structure of ν5 from the mm/submm analysis is of particular interest because it is one of the bands that is used for atmospheric remote sensing.