Ahmed Agayev, better known as Ahmet Ağaoğlu (1869-1939), has been a prominent preacher of Turkism and one of the founding fathers of the so-called Azerbaijani identity, having played also a significant role in the formation of Pan-Turkism. Ağaoğlu’s involvement in Pan-Turkist circles in the Ottoman Empire and then in the nationalist movement in Kemalist Turkey partly overshadowed some details of his earlier life. This paper examines one of the lesser-known episodes in his biography—his participation in the activities of the Iranian revolutionaries in Istanbul and his collaboration with their Persian organ, Sorush (Sorūš) in 1909-1910 in Istanbul. Ironically, positioning himself in his Persian writings in Sorush as an avid follower of Iranian nationalism, Ağaoğlu began soon to propound in the Ottoman press the idea of “the Turks of Iran”, actively promoting Turkism and Pan-Turkist views on the ethnic background of the South Caucasian Muslims and the population of the northwestern areas of Iran. Ahmet Ağaoğlu’s writings in Istanbul in 1909 and 1910 shed some light on the genesis of a modern ethnic identity, which was later labelled as “Azerbaijani”.