17 Thetis (thee'-tis) is a large Main belt asteroid. It is a S-type asteroid, so it has a relatively bright surface and stony silicate and metallic iron-nickel composition.

It was discovered by R. Luther on April 17, 1852. It was his first asteroid discovery. Its name comes from Thetis, the mother of Achilles in Greek mythology.

One stellar occultation by Thetis is observed from Oregon in 1999. However, the event was not timed.