The Turkmanchai treaty (also written Turkemanchay, Turkamanchay, and Turkmanchay) is a treaty by which Iran lost many of its northern territories to Imperial Russia after its defeat in 1828, bringing an end to the second Russo-Persian wars (1827-1828).

The treaty was signed on February 21, 1828 (5th of Shaban, 1243 in the Islamic calendar), by Haj Mirza Abol-hasan Khan and Asef o-dowleh , chancellor of Fath Ali Shah from Iran's side, and General Ivan Paskievich representing Imperial Russia.

As was the case for the Gulistan Treaty, Iran was forced to sign the treaty by Russia, as it had no other alternative after Abbas Mirza's defeat. The Russian general had threatened Fath Ali Shah to conquer Tehran in five days unless the treaty was signed.

By this treaty:

1. By Article 4 of the treaty, Iran loses sovereignty over Yerevan (current capital of Armenia), Nakhichevan, Talysh, Ordubad, and Mughan regions (now part of Azerbaijan Republic), in addition to all lands annexed by Russia in the Gulistan Treaty.
2. The Aras river is assigned the new borderline between Iran and Imperial Russia, from "from Ararat ghal'eh to the outlet of the river at Astara".
3. By Article 6 of the treaty, Iran promised to pay Russia 10 Koroor in Gold (in 1828 currency).
4. By Article 8 of the treaty, Iranian ships lose full rights to navigate all of the Caspian Sea and her coasts, henceforth given to Russia.
5. Iran recognizes Capitulation rights for Russians in Iran.
6. By Article 10, Russia gains the right to send consulate envoys to anywhere in Iran it wishes.
7. By Article 13, Exchange of POWs.
8. By Article 10, Iran is forced to sign economic treaties with Russia as Russia specifies.
9. By Article 7 of the treaty, Russia promises to support Abbas Mirza as the heir to the throne of Persia after Fath Ali Shah dies. (which did not happen).
10. Iran officially apologizes for breaking its promises made in the Gulistan Treaty.
11. By Article 15, Fath Ali Shah promises not to prosecute any Khanate secessionist movements in the Azerbaijan region.

Iran officially sees this and the preceding Gulistan Treaty as one of Iran's most humiliating treaties ever signed. The treaty is also regarded by Iranians as the main reason why Fath Ali Shah is seen as one of Iran's most incompetent rulers in memory.