'Ammar b. Yasir (Arabic: عمار بن ياسر) (d. 37/657) was an early companion of Prophet Muhammad (s) and among the first people to become Muslim. His family was brutally tortured for believing in the Prophet (s) and this resulted in his parents' martyrdom. 'Ammar b. Yasir had the kunya of Abu Yaqza.
'Ammar, along with Salman, Miqdad, and Abu Dhar, is considered to be one of the first Shi'a. He was among those few Muslims who refused to pay allegiance to Abu Bakr. In one incident, after criticizing the third caliph for unlawful spending from the public treasury, the caliph kicked him so badly that he fell unconscious. The caliph planned to banish him, but then withdrew due to Imam 'Ali's (a) mediation.
'Ammar b. Yasir was an advocate of Imam 'Ali's (a) caliphate. When 'Umar died and the Six-Member Council was held to select the caliph, he talked with 'Abd al-Rahman b. 'Awf and recommended him to choose 'Ali (a) as the caliph in order to prevent divisions among people. When 'Uthman was killed, 'Ammar was one of the people who invited others to pledge their allegiance to 'Ali (a).
During the caliphate of Imam 'Ali (a), 'Ammar attended battles of Jamal and Siffin. In the Battle of Jamal, he was the commander of the left army of Imam 'Ali (a). On the third day of the Battle of Siffin, he was also the commander of Imam 'Ali's (a) army.