Agsu (also, Ağsu, Aghsu, Akhsu and Aksu) (Urdu: آق سو) is a city in and capital of the Agsu Rayon of Azerbaijan.
A. A. Bakikhanov noted that Nadir Shah issued an order for resettlement of the inhabitants of Shamakhi to new laid down city on the bank of Agsu River, in 1735. At present, the name of Agsu city (which is the centre of Agsu district) is shown as New Shamakhi in some sources. It is related with resettlement of the inhabitants of Shamakhi to Agsu city. Since that time, historic Shamakhi has been called Old Shamakhi, Agsu – New Shamakhi.
The name of the city existing in 18th century was frequently shown as Agsu deriving from "Agsuchay". "Ruined city" – ruins of the Agsu city of the memory of 18th century is located in the north of Agsu city.
S. Bronevski noted that the city was fenced with fortress walls. Castle had quadrangle and round towers. There was dug a trench in front of fortress walls. Lower part of dwelling building consisted of basements. Upper part of dwelling building tumbled down onto the basements. These holes are the remainders of the basements. Dwelling buildings were erected very close to each others. Streets were narrow, transport with wagons were not possible. Undoubtedly, central streets comparatively were wide.
Earthenware of the Middle Ages and a coin minted named after Surkhay Khan were found in Agsu city. The coin had been minted before Nadir Shah's order about laying out the city. But it raises the probability of creation of Agsu city on base of Agsu village. At the same time, materials and ceramic goods belonging to 100–150 years before occupation of Shamakhi by Nadir Shah were found during initial researches in "Ruined city". Perhaps, Nadir Shah had issued an order for realization of additional works after resettlement of the inhabitants in the village.
Agsu city had durable fortress walls, defensive towers in 18th century. The city had gradually developed and it had become one of the largest cities of Azerbaijan in the end of 18th century. V. Leviatov noted that when population of New Shamakhi (Agsu) had reached nearly 10 thousand, population of Gandja, Ardabil, Shusha, Nukha, Baku and other cities had not been over 3-8 thousand.
Agsu city is the one of the important cities leading in social-political and military life of Azerbaijan in the end of Middle Ages. In 1734, Nadir Shah attacked Shamakhi with his troop. The inhabitants of Shamakhi resisted against the attack, notwithstanding Surkhay khan had left the city. In August 1734, Nadir Shah's troop overcame the resistance and the city was burnt. In 1735, Nadir Shah issued an order about laying out the new city on the bank of Agsuchay River. After construction of the new city, the inhabitants of Shamakhi were resettled here; residences of rulers of Shamakhi also were removed here.
In 1743, inhabitants rebelled against the Nadir Shah's yoke. Liar princes of Safaviler – I, II, III Sam Mirzas came out. Two of them operated in Shirvan. I Sam Mirza occupied New Shamakhi (Agsu) to the accompaniment of Surkhay Khan's troop of fifty thousand. He revoked taxes imposed by Nadir Shah. But, I Sam Mirza was killed in the war by the son of Nadir Shah Nasrullah Mirza.
II Sam Mirza made a great rebellion in Shirvan together with Mohammad (the son of Surkhay), in 1743. People of Agsu banished Iranian officials from the city. But, despite of three defeats, Nasrullah occupied Agsu castle. Based on source, Persians encircled the city. The city was occupied after some days later and people were killed. Nadir Shah had called this day "Allah dad" (God, help me).
There were organized independent khanates after the death of Nadir Shah (1747). The one of the khanates was Shirvan (1747–1763) which its supremacy was diarchy. At this time, feudal lords struggled against each others in Azerbaijan, in 1755, Khan of Shaki Hazhi Chalabi Khan encircled Agsu city under existing conditions. Hazhi Chalabi Khan retreated after grievous loss in the war around Agsu. Hussein Ali Khan went back to his khanate, notwithstanding Hazhi Mohammad Ali Khan had given the key of the city to him. In 1762, Transcaucasia was shrouded in plague.
In 1763, Agsu and Shamakhi were amalgamated. Mohammad Said Khan and Agasi Khan occupied Agsu, and then killed Hazhi Mohammad Ali Khan. The capital was moved from Agsu to Shamakhi. In 1767, Fatali Khan (Khan of khanate Guba) and Hussein Khan (Khan of khanate Shaki) joined and encircled together to Shamakhi from two sides. But Mohammad Said Khan and Agasi Khan were not able to resist. Consequently, the capital was removed to Agsu. Sardar and Hassan estates of Shirvan were amalgamated to khanate Shaki, remained estates were amalgamated to khanate Guba.
In 1769, Agsu city was ruined by the troop of Fatali khan (khan of khanate Guba), the inhabitants were removed to Shamakhi. Based on sources, all inhabitants were settled down in Shamakhi in a brief time. S. Gmelin noted that when he was in Agsu he could not find anything except destroyed buildings.
Based on written information of travelers M. Biberstein, S. Bronevski and others, some streets remained deserted when Fatali Khan forced the inhabitants to deportation. Agsu city was exposed to attacks the next years. Especially, when Aga Mohammad Shah Gajar's marched to Azerbaijan, the survived inhabitants immigrated to Guba and other regions, non-survived were annihilated. So, Agsu city which was founded by Nadir Shah, was decayed by Aga Mohammad Shah Gajar.
Agsu city was repeatedly exposed to feudal attacks in 18th century. Frequent deportation, destructions made the inhabitants disappointed in living in the city. Therefore, the inhabitants did not prepare to construct glamorous buildings, tombs, mosques, bathrooms, social and official buildings. In 1967, Agsu was entitled with city-type settlement status, in the same year it was entitled with city status.
Agsu has been operated as administrative-territorial district since October 8, 1943.