In mid-1996, the Multiple Mirror Telescope (MMT) will be upgraded from its current configuration of six 1.8-meter primary mirrors to house a single 6.5-meter primary. To exploit the dramatic increase in collecting area and spatial resolution, as well as the high infrared quality of the observing site, we have designed a new spectrometer that will cover the thermal infrared (4 to 25 micrometer) at high spectral and spatial resolution. The design is based on the new generation of Si:As BIB detectors with 256 by 256 formats, which represent a substantial advance in array size at these wavelengths. The design consists entirely of reflecting optics, and is arranged to deliver diffraction-limited resolution longwards of 8 micrometers. A maximum spectral resolving power of R equals 15,000 is provided by an echelle grating. A lower resolution grating or a plane mirror for two-dimensional imaging may be substituted for the echelle.