The Mid-Infrared Spectrometer (MIRS) is one of four instruments that will fly aboard the orbiting Infrared Telescope in Space (IRTS). This telescope is a joint NASA/Japanese Space Agency (ISAS) project that is scheduled for a Spring, 1995, launch aboard a Japanese expendable launch vehicle and subsequent retrieval by the space shuttle. The telescope itself is liquid helium-cooled with a 15 cm aperture and will survey approximately 10% of the sky before its cryogen runs out and it begins to warm up. The MIRS was developed jointly by NASA, the University of Tokyo, and ISAS and operates over a wavelength range of 4.5 to 11.7 microns with a resolution of 0.23 and 0.36 microns. The MIRS has a conventional entrance aperture, so that spectral studies can be made of extended as well as point-sources. A cold shutter and an internal calibrator allow accurate absolute flux determinations. Calibration and sensitivity tests in the laboratory have shown that the instrument sensitivity will be limited by the fluctuations due to the zodiacal dust emission over the wavelength range of the spectrometer. The large A-omega of the spectrometer, the cryogenic optics, and the survey nature of the telescope will allow very sensitive studies of the spectral characteristics of diffuse extended emission. These observations will help in determining the composition of the galactic dust responsible for the warm component of the infrared cirrus. In secondary observing programs, the MIRS will also take spectra of the zodiacal dust emission as well as measure the infrared spectra of an estimated 9,800 point-source objects.