Ithaca Chasma is a huge valley on Saturn's moon Tethys. It is 100 km wide, 3 to 5 km deep and 2,000 km long, running approximately 3/4 of the way around Tethys' circumference. It is named after the island of Ithaca, in Greece.

It is thought that Ithaca Chasma formed as Tethys' internal liquid water solidified, causing the moon to expand and cracking its surface to accommodate the extra volume within. Earlier craters made before Tethys solidified were probably all erased by geological activity before then.

An alternative theory is that it was formed at the same time as the enormous crater Odysseus which is on the oposite side of the moon. When the impact that created Odysseus occurred, the shockwave may have traveled through Tethys and fractured the icy, brittle surface on the other side.