The Thermal Emission Imaging System (THEMIS) is an instrument on board the Mars Odyssey spacecraft. It combines a 5-wavelength visual imaging system with a 9-wavelength infrared imaging system.

The orbiter launched from Kennedy Space Center on April 7, 2001 and arrived at Mars on October 24, 2001. The orbiter spent the next several months achieving a circular mapping orbit by aerobraking (dipping into the atmosphere to slow and shrink the orbit). Aerobraking concluded in early February 2002, and primary mapping operations began a few weeks later.

Mars Odyssey carries three main science instruments: The Gamma Ray Spectrometer (GRS), the Thermal Emission Imaging System (THEMIS), and the Mars Radiation Environment Experiment (MARIE).