Jamshid ([dʒæmˈʃiːd]) (Persian: جمشید, Jamshēd; Middle- and New Persian: جم, Jam), also known as Yima (Avestan: 𐬫𐬌𐬨𐬀 Yima; Persian/Pashto: یما Yama), is the fourth Shah of the mythological Pishdadian dynasty of Iran according to Shahnameh.

In Persian mythology and folklore, Jamshid is described as the fourth and greatest king of the epigraphically unattested Pishdadian Dynasty (before the Kayanian dynasty). This role is already alluded to in Zoroastrian scripture (e.g. Yasht 19, Vendidad 2), where the figure appears as Yima xšaēta (Avestan: 𐬫𐬌𐬨𐬀⸱𐬑𐬱𐬀𐬉𐬙𐬀) "radiant Yima", from which the name 'Jamshid' is derived.

Both Jam and Jamshid remain common Iranian and Zoroastrian male names that are also popular in surrounding areas of Iran such as Afghanistan and Tajikistan. Edward FitzGerald transliterated the name as Jamshyd. In the eastern regions of Greater Iran, and by the Zoroastrians of the Indian subcontinent it is rendered as Jamshed based on the Classical Persian pronunciation.

The name Jamshid is originally a compound of two parts, Jam and shid, corresponding to the Avestan names Yima and Xšaēta, derived from the Proto-Iranian *Yamah Xšaitah ('Yama, the brilliant/majestic'). Yamah and the related Sanskrit Yama are interpreted as "the twin", perhaps reflecting an Indo-Iranian belief in a primordial Yama and Yami pair. By regular sound changes (y → j, and the loss of the final syllable) an Old Persian form equivalent to Avestan Yima became Middle Persian Jam, which was subsequently continued into New Persian.

There are also a few functional parallels between Avestan Yima and Sanskrit Yama; for instance, Yima was the son of Vivaŋhat, who in turn corresponds to the Vedic Vivasvat, "he who shines out", a name for the sun-god Surya. Both Yamas in Iranian and Indian myth guard Hell with the help of two four-eyed dogs.

Oettinger, in the course of voicing his contention that the story of Yima was originally a flood myth, and that original Sanskrit flood myths had as their protagonist Yama, mentions that Scandinavian mythology features a cognate of Yima and Yama, in the form of the primordial being Ymir, a giant from whose corpse the cosmos was fashioned and the shedding of whose blood caused a great flood. Ymir was still associated with a flood in mediaeval Iceland, Snorri Sturluson recording that his pagan forbears had believed that the death of Ymir had caused a great flood, killing all but one of the Frost Giants. This sole named survivor was Bergelmir, who - Noah-like - escaped drowning with his wife by floating on a lúðr (wooden vessel of some kind) and so lived on to perpetuate his race. The role of Bergelmir in the myth of Ymir may have been influenced by Christian mythology,but the association of Ymir with a great flood may be attributable to an authentic pre-Christian tradition. Oettinger's main argument concerned the way in which Indic "Yama" and the Iranic "Yima" - both cognates of the Norse "Ymir" - also related to the flood.

*Xšaitah meant "bright, shining" or "radiant". By regular sound changes (initial xš → š (sh); ai → ē; t → d between vowels; and dropping of the final syllable) *xšaitah became Persian shēd. In Iranian Persian, the vowel /ē/ is pronounced as /i/. Consequently, Jamshēd (as it is still pronounced in Afghanistan and Tajikistan) is now pronounced Jamshid in Iran. The suffix -shid is the same as that found in other names such as khorshid ("the Sun" from Avestan hvarə-xšaēta "radiant Sun").

The modern Turkish name Cem is derived from the Persian Jam.