Themisto (the-mis'-toh, Greek Θεμιστώ) is a moon of Jupiter. It was first discovered by Charles T. Kowal and Elizabeth Roemer on September 30, 1975, reported on October 3 (IAUC 2845) and designated S/1975 J 1, but not enough observations were made to establish an orbit and it was subsequently lost.

It appeared as a footnote in astronomy textbooks into the 1980s. Then, in 2000, a seemingly new satellite was discovered by Scott S. Sheppard, David C. Jewitt, Yanga R. Fernández and Eugene A. Magnier , and was designated S/2000 J 1. It was soon confirmed that this was the same as the 1975 object. The Sheppard et al. announcement (IAUC 7525, November 25, 2000) was immediately correlated with an August 6, 2000 observation by the team of Brett J. Gladman, John J. Kavelaars, Jean-Marc Petit , Hans Scholl, Matthew J. Holman , Brian G. Marsden, Philip D. Nicholson and Joseph A. Burns —an observation that was reported to the Minor Planet Center but not published as an IAU Circular (IAUC).

It is also designated as Jupiter XVIII.

In 2002 it was officially named after Themisto, a lover of Zeus (Jupiter) in Greek mythology. Themisto's orbit is unusual, as it orbits about midway between the Galilean moons and the first group of prograde irregulars. It is about 8 km in diameter.