Iso-propanol [(CH_3_)_2_CHOH], an isomer of n-propanol, has been studied in the millimeter- and submillimeter-wave region of the electromagnetic spectrum with our FASSST spectrometer through 360GHz. Spectra arising from the ground vibrational state of all three hydroxyl torsional substates, given the labels symmetric gauche, antisymmetric gauche, and trans in order of increasing energy, have been observed. We have successfully assigned ~7600 pure rotational transitions within the torsional substates as well as ~4700 torsional-rotational transitions between the symmetric and antisymmetric gauche substates through the lower rotational quantum number J"=68. Spectral lines involving one or both of the two gauche forms have been simultaneously analyzed with a 2x2 effective torsional-rotational Hamiltonian, which includes terms through fifth order in the torsional-rotational interaction. Excluding perturbed transitions, the assigned transitions were fitted to a root mean square deviation of 76kHz. The trans substate was analyzed as a semirigid rotor, and its unperturbed transitions fitted to a root mean square deviation of 63kHz. A perturbation was seen at transitions with J">50 in the trans substate. The torsional excitation energy for the trans substate above ground was estimated from intensity ratios to be about 120K.