The Persian Cossack Brigade, also known as the Iranian Cossack Brigade (Persian: بریگاد قزاق, romanized: Berīgād-e qazzāq), was a Cossack-style cavalry unit formed in 1879 in Iran. It was modelled after the Caucasian Cossack regiments of the Imperial Russian Army. Until 1920, it was commanded by Russian officers, while its rank and file were composed of ethnic Caucasians and later on Iranians as well. During much of the brigade's history it was the most functional and effective military unit of the Qajar dynasty. Acting on occasion as kingmakers, this force played a pivotal role in modern Iranian history during the Persian Constitutional Revolution, the rise of Reza Shah, and the foundation of the Pahlavi dynasty.

The Cossack Brigade was formed by Naser al-Din Shah in 1879, using as a model the Caucasian Cossack regiments of the Imperial Russian Army which had impressed him when travelling through southern Russia in 1878. During his trip to Europe in 1878–1879, the shah had stayed in Yerevan, where a party of Cossacks had escorted him and the rest of the Iranian court across Russian Armenia. Naser al-Din expressed much admiration for the colorful uniforms of the Cossacks along with their skilled horsemanship and their talents with their guns and shashkas (the type of sword favored by the Cossacks). In Tiflis (Capital of Russian Caucasus Viceroyalty), Naser al-Din asked the Governor-General Grand Duke Mikhail for Russian assistance in settling up a Cossack unit in Iran. In turn, the Duke passed along the request to Emperor Alexander II in St. Petersburg, where it was approved. Together with a Swedish-trained and officered gendarmerie, the Cossack Brigade came to comprise the most effective military force available to the Iranian crown in the years prior to World War I.

In spite of its name the brigade was not a typical Cossack force as employed in the neighbouring Russian Empire. The Cossack regiments of the Imperial Russian Army were based on a feudal-style system under which military service was given in return for long-term grants of land. By contrast the Persian Cossack Brigade was recruited on a conventional basis, from a mix of volunteers and conscripts. Neither did it have the status of a guards unit. However, it closely resembled a true Cossack-style cavalry unit. Late 19th century photographs (see example below) show Russian style uniforms, in contrast to the indigenous dress of other Iranian forces at the time.

Russian attitudes towards Iran in the 19th century were a mixture of patronising contempt for an "Oriental" country along with a genuine respect for Qajar shahs as fellow monarchs who were the rightful rulers of Iran. In spite of the religious divide between Orthodox Russia and Muslim Iran, there was a sense of monarchial solidarity held by elites in both nations that regarded all monarchs regardless of their religion to be entitled to respect and obedience from their subjects. Though Russian-Iranian relations in the 19th century were far from harmonious, the Qajar shahs-most notably Naser al-Din-who ruled as absolute monarchs had an instinctive preference for Russia, whose autocratic political system was so close to their own. The so-called "Great Game" as competition for influence in Asia between the British and Russian empires had the effect of preserving Iranian independence at a time when Iran was becoming dangerously close to becoming to use a modern term a "failed state". Neither the British nor the Russians would tolerate Iran being annexed to the other rival empire, and so both powers acted to preserve Iran as a state. There was a gradual breakdown in the authority of the Iranian state during the later Qajar era as corruption was so rampant to the point of pushing Iran to the brink of bankruptcy. Corruption was epidemic during the reign of Naser al-Din. The shah held semi-annual auctions where all the offices of the Iranian state were sold to the highest bidder and in turn were allowed to plunder whatever office they had purchased. Only loans from Russia and Britain prevented Iran from actually going bankrupt, and in return both Britain and Russia received special rights in Iran. Naser al-Din's brother famously compared Iran to "a lump of sugar in a glass of water" that was slowly melting away as corruption hallowed out the Qajar state. Based on the way that the Iranian state was dissolving in the later Qajar era to the rampant corruption, Iran would have almost certainly been annexed to either to the British or the Russian empires, and only the fact that neither empire considered it acceptable to see Iran annexed to the other rival empire prevented Iran from being annexed. Russian policy in Iran was to preserve Iran, but at the same time to keep Iran so weak that it would never be capable of challenging Russia. In this context, providing Naser al-Din with a Cossack brigade that was properly trained, paid and armed would ensure that the Shah was strong enough to keep the Peacock Throne, but never strong enough to challenge Russian influence in Iran.

The rank and file of the brigade were always Caucasian Muhajir and later Iranians as well, but until 1920 its commanders were Russian officers who were also employed in the Russian army, such as Vladimir Liakhov. Such secondments were encouraged by the Imperial Russian Government who saw the Cossack Brigade as a means of extending Russian influence in a key area of international rivalry. After the October Revolution in 1917, many of these Russian officers left the country to join the "White" forces. The command of the Persian Cossack Division was subsequently transferred to Iranian officers. Most notable among these officers was General Reza Khan, who started his military career as a private soldier in the Cossack Brigade and rose through its ranks to become a brigadier general.

At the time of the Persian Cossack Brigade's formation the Shah's royal cavalry was described as having no training or discipline. The Qajar state at this point was very weak, lacking any professional military force. In wars against the British, the royal cavalry had been defeated; and had even had much difficulty fighting against Turcoman nomads. Tsar Alexander II approved Russian military advisors traveling to Iran to fulfill the Shah's request for assistance. The brigade was then formed in 1879 by Lieutenant-Colonel Aleksey Domantovich, a Russian officer.

Russian interests and lack of funding slowed the initial development of the brigade into a professional fighting force. The initial strength of the brigade was 400 men drawn from immigrants known as Caucasian muhajirs, who were descendants of Circassians and Transcaucasian Muslims who had migrated to Iran to avoid Russian rule. They possessed special privileges as a hereditary military caste. Domantovich made rapid progress with their training and the Shah ordered the strength of the new brigade to be increased to 600 men drawn from the regular army. The rapid progress of the Cossack Brigade caused concern in Russia due to the fear that it might become a true fighting force rather than a tool of the Russian government. Domantovich complained that the Imperial Iranian Army had no quartermasters, which forced him to buy supplies and uniforms from Tehran merchants for the Cossack Brigade out of his own salary. The fact that Domantovich served as the brigade's quartermaster as he purchased all supplies allowed him to dominate the brigade, but it seemed that this situation came about more of necessity as the Shah was too cheap to establish a quartermaster corps in his army as opposed to a plan to establish Russian influence. Domantovich was an honest officer who dismissed several of his Persian officers for "taking liberties" with the brigade's finances as he phrased it, which greatly contributed to the professionalism of the brigade, which was the only unit in the Iranian Army that was paid on time and in full. The Iranian historian Kaveh Farrokh wrote about the Cossack brigade: "Domanovich introduced a number of positive practices including strict discipline, prompt pay, and promotion through military merit rather than family/political connections".

Domantovich was dismissed as commander in 1881 and replaced by the less effective Colonel Charkovsij, over the protests of the Shah. Domantovich had come into conflict with the Russian minister-plenipotentiary in Tehran, I.A. Zinoviev, who saw him as a rival as Domantovich reported directly to Naser al-Din. In his reports to St. Petersburg, Zinoviev accused Domantovich of damaging Russia's interests in Iran by ironically establishing an effective military unit which might one day be used against Russia. Zinoviev argued that the Cossack Brigade should be used as a tool for extending Russian influence in Iran, but not to the point of giving the Shah militarily effective unit.