Ammar ibn Yasir (Arabic: عَمَّار بْن يَاسِر, romanized: ʿAmmār ibn Yāsir; c. 567/570 – July 657 C.E.) was a Sahabi (Companion) of the Islamic prophet Muhammad, and a commander in the early Muslim conquests. His parents, Sumayya and Yasir ibn Amir, were the first martyrs of the Ummah. Ammar converted to Islam by the invitation of Abu Bakr and was amongst the muhajirun. After the migration to Medina, he participated in building the Prophet's Mosque and fought in most of the early Muslim expeditions.

He fought in the Ridda wars under Caliph Abu Bakr (r. 632–634) and in the Muslim conquest of Iran under Caliph Umar (r. 634–644). Ammar served as governor of Kufa under Umar. Following Uthman's assassination, Ammar became a devout partisan of Caliph Ali (r. 656–661) and died while fighting on Ali's side in the Battle of Siffin.

Ammar belonged to the Malik clan of the Madhhij tribe in Yemen. He later lived in the Hijaz region (in present-day Saudi Arabia). Born in or around the Year of the Elephant—the same year as Muhammad—he grew up in Mecca and served as one of the intermediaries in Muhammad’s marriage to Khadijah bint Khuwaylid. His father, Yasir ibn Amir, came from the tribe of Qahtan in Yemen and migrated to Mecca, where he settled after marrying Sumayya, a slave woman. Ammar and his parents, Yasir and Sumayya, were slaves to Abu Huzaifa, and after his death they came under the control of Abu Jahl, who later became one of Islam’s fiercest enemies and the notorious torturer of the family. Ammar’s trust in and knowledge of Muhammad's credibility—even before his prophethood—encouraged him to follow Muhammad's prophetic visions as one of the earliest converts.

Belonging to the Banu Makhzum, Ammar was born in 567 to Yasir ibn Amir and Sumayya, both of whom were later brutally killed. At an early age, Ammar converted to Islam by the invitation of Abu Bakr, becoming one of the earliest Muslims. He became one of Muhammad’s most prominent companions, participating in all his military campaigns and battles. Historically, Ammar ibn Yasir was the first Muslim to build a mosque.

ʿAmmār converted to Islam in 614 or 615 CE. This was during the period when the Quraysh were persecuting lower-class Muslims. As Ammar later recounted to his grandson: "I met Suhayb ibn Sinan at the door of the house of Al-Arqam while the Messenger of Allah was inside. I asked him, 'What do you want?' He replied, 'What do you want?' I answered, 'I want to go to Muhammad and listen to what he says.' He said, 'That is what I want.' We entered, and he presented Islam to us. We became Muslim and spent the day until evening, then went out concealing ourselves". Ammar's father, mother, and brother also became Muslims, though not at Abu Bakr's invitation.

When the Quraysh learned of Yasir's family's conversion, they became among "the victims who were tortured at Mecca to make them recant". The Makhzum clan would expose Ammar, his father, and his mother to the scorching heat of the day and torture them over open fire. Muhammad would pass by and say, "Patience, O family of Yasir! Your meeting-place will be Paradise" and "O fire! Be cool and harmless for ‘Ammar in the same manner in which you became cool and harmless for Ibrahim.” As a result, Ammar bore scars from this torture for the rest of his life.

Ammar was tortured "until he did not know what he was saying," as was his friend Suhayb. In this state, he temporarily maligned Muhammad and praised the pagan gods. Afterwards, he confessed to Muhammad. When asked, "How do you find your heart?" Ammar replied that he was still a Muslim at heart. Muhammad said all was well. The Qur'an verse "someone forced to do it whose heart remains at rest in its faith" (16:106) refers to Ammar. His mother was murdered by Abu Jahl for refusing to abandon Islam and is regarded as the first Muslim martyr. The opening verses of Surat Al-Ankabut (chapter 29: The Spider) were revealed in response to this event.

To escape the Meccans' torture, ibn Saad and Ibn Ishaq report that Ammar went to Abyssinia in 616.