Nothing is known with certainty about Ertuğrul's life, other than that he was the father of Osman; historians are thus forced to rely upon stories written about him by the Ottomans more than a century later, which are of questionable accuracy. According to many Turkish sources, Ertuğrul had three brothers named; Sungur-tekin, Gündoğdu and Dündar. After the death of their father, Ertuğrul with his mother Hayme Hatun, Dündar and his followers from the Kayı Tribe migrated west into Anatolia and entered the Seljuk Sultanate of Rum, leaving his two brothers who took their clans towards the east. In this way, the Kayı Tribe was divided into two parts. According to these later traditions, Ertuğrul was chief of his Kayı Tribe. As a result of his assistance to the Seljuks against the Byzantines, Ertuğrul was granted lands in Karaca Dağ, a mountainous area between Diyarbakır and Urfa, by Kayqubad I, the Seljuk Sultan of Rum. One account indicates that the Seljuk leader's rationale for granting Ertuğrul land was for Ertuğrul to repel any hostile incursion from the Byzantines or other adversary. Later, he received the village of Söğüt which he conquered together with the surrounding lands. That village, where he later died, became the Ottoman capital under his son, Osman I. Osman's mother has been referred to as Halime Hatun in later myths, and there is a grave outside the Ertuğrul Gâzi Tomb which bears the name, but it is disputed.

According to many sources, he had two other sons in addition to Osman I: Saru-Batu (Savci) Bey and Gündüz Bey. Like his son, Osman, and their descendants, Ertuğrul is often referred to as a Ghazi, a heroic champion fighter for the cause of Islam.