Kayumarth I (also spelled Gayumarth I or Kayumars I; Persian: ملک کیومرث یکم) was the ruler (ustandar) of the Baduspanids from 1394 to 1453, with a three-year interruption.

After his death, a dynastic struggle followed, which resulted in his kingdom being split up by his sons Iskandar IV and Ka'us II, in Kojur and Nur respectively.

The Baduspanids were a local Iranian dynasty that ruled the mountainous district of Rustamdar in western Mazandaran, a region on the Caspian coast of northern Iran.

[3] During the time of Kayumarth, the Caspian region of northern Iran was under the control of several local dynasties who often intermarried, fell into dispute over lands, and intervened in each other's affairs.

The Baduspanids had been temporarily removed from power after the ustandar Adud al-Dawla Qubad (r. 1379–1381) was defeated and killed in 1381 by the Mar'ashis, who incorporated Rustamdar into their own domains.

"[1] When Iskandar-i Shaykhi left his realm in 1399/1400 to join Timur's Azerbaijan expedition, the latter deprived the Baduspanids of most of their holdings by sending his troops to administer most of Rustamdar.

[2][1] Iskandar-i Shaykhi was defeated and killed in 1403/4 at Shir-rud-duhazar, and Timur appointed the Mar'ashi Sayyid Ali Sari as the governor of Amul, with his brother Ghiyath al-Din as his second-in-command.

When he reached Nur, he killed its Timurid commander, and subsequently restored Baduspanid rule in Rustamdar with the assistance of its inhabitants.

He made incursions into the domains of Ilyas Khwaja (a vassal of Shah Rukh) to the south of the Alborz, attacking Simnan and Bistam, while capturing the castle of Tabarak, near Ray.

Shah Rukh eventually sent a force under Abd al-Ali Bakavli to warn Kayumarth and aid Ilyas if required.

At the behest of Shah Rukh, Kiya Muhammad returned Kayumarth's territories to him, with the exception of Taleqan and the castle of Falis.

However, Kayumarth soon made an agreement with Sayyid Muhammad, in which he agreed to withdraw his support for the latter's enemies in return for control over Mianrud.

After his death, a dynastic struggle followed, which resulted in his kingdom being split up by his sons Iskandar IV and Ka'us II, in Kojur and Nur respectively.

[1] The Baduspanid dynasty was never to be united again, with the two branches ruling separately until they were eventually deposed in the 1590s by the Safavid monarch of Iran, Abbas the Great (r. 1588–1629).

In the rural parts of Rustamdar, he had several shrines constructed on top of the tombs of Shia scholars and descendants of the prophet.