The Sithi (called Zida'ya in their own tongue) are a cryptic, near-immortal elder race from the "Memory, Sorrow, and Thorn" trilogy by Tad Williams.

As tall but far more slender than humans in Williams books, the Sithi are skilled with bows, are lithe of build, typically have golden, tanned skin, and may or may not have pointed ears. They are allergic to iron, and many of their weapons are made from a substance called "witchwood", which grows as a tree but is then forged and worked like metal. Their behavior toward mortals, when they have any contact at all, is courteous but very aloof. Williams describes them as an almost feral race. Williams' idea of the Sithi was influenced by the Irish legend of the Sidhe, and by Tolkien's elves. However, Williams gives them a Japanese slant: many of their names are based upon Japanese words, such as nihongo|Jiriki|自力|"jiriki"|"by oneself" and Amerasu (from Amaterasu).

In Williams' series, the Sithi are said to have come from "over the sea", having lived in The Garden prior to a cataclysm that drove them into exile. They now live secluded in the forests, far away from the younger human race, which has persecuted and hunted them in the past. An embittered tribe of "Sithi", named Norns (in the Garden-tongue "Hikeda'ya"), live in the far north, seeking vengeance upon all Mankind. A close cousin of the Zida'ya and the Hikeda'ya, the Tinukeda'ya (or "Ocean Children"), are the Niskies and the Dwarrows, which, although of the same family, are drastically different from one another. Other isolated "Sithi" tribes lie scattered across the face of Williams' world, Osten Ard, but it is clear the entire race has dwindled to near non-existence.

The Sithi are often described in animalistic terms. Frequently, they are referred to as "cat-like" or "bird-like" in their movements or gestures; their northern kin are called "white foxes". These references accentuate the differences between the Sithi and Mankind.

The Sithi and their kin naturally have white hair, but the Sithi typically dye it in bright shades of color. Lavender-blue, sky-blue, yellow, red, gray, nut-brown, grass-green, and black have all been specifically mentioned. A few Sithi sport multicolored hair. The Norns do not dye their hair at all.

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