The Massacre of Bi'r Ma'una (Arabic: يَوْم بِئْرِ مَعُونَةَ, romanized: Yawm Biʾr Maʿūna, lit. 'Day of Bi'r Ma'una') was the killing of Muslim missionaries by the Mushrikites four months after the Battle of Uhud in 625 (4 AH). It is believed the Islamic prophet Muhammad sent missionaries to preach Islam, at the request of Abu Bara. Forty (as per Ibn Ishaq) or seventy (as per Sahih Bukhari) of the Muslim missionaries were killed.

Four months after the Uhud battle, a delegation of Banu Amir came to Muhammad and presented him with a gift. Abu Bara stayed in Medina. Muhammad declined to accept that gift because it was from a polytheist and asked Abu Bara to embrace Islam. He requested Muhammad to send some Muslims to the people of Najd to call them to Islam. At first, Muhammad was quite apprehensive of this, as he feared that some harm might befall these Muslim missionaries. On Muhammad’s hesitation, Abu Bara guaranteed the safety of the emissaries of Muhammad.

The Muslim scholar al-Tabari describes the event as follows:

The Messenger of God declined to accept it, saying, "Abu Bara', I do not accept presents from polytheists, so become a Muslim if you want me to accept it." Then he expounded Islam to him, explained its advantages for him and God's promises to the believers, and recited the Qur'an to him. He did not accept Islam, but was not far from doing so, saying, "Muhammad, this matter of yours to which you call me is good and beautiful. If you were to send some of your companions to the people of Najd to call them to your religion, I would hope that they would respond to you." The Messenger of God said, "I fear that the people of Najd would do them some harm." Abu Bara' replied, "I will guarantee their protection, so send them to call people to your religion." The Messenger of God thereupon sent al-Mundhir b. `Amr [Tabari Volume 7, p. 151]

Ibn Ishaq's Biography claims that forty men were sent to them; but Sahih al-Bukhari states that there were 70— Al-Mundhir ibn Amr, one of Banu Sa‘ida, nicknamed ‘Embraces Death’ — commanded that group, who were the best and most learned in the Qur'an and jurisprudence.

A short time after the Raj’i incident (4 Safar/April 625), chief of the Amir ibn Sa'sa' tribe Abu Bara' Amir ibn Malik came to Madina and obtained information about Islam from Muhammad. Despite not being Muslim himself, he requested that Muhammad send representatives to his tribe to teach them about Islam. After receiving assurances of their safety, Muhammad assigned a group of seventy (or forty) people well-versed in the Qur’an, most of them natives of Madina and people of the Suffa, led by Mundhir ibn 'Amr al-Khazraji. When the group arrived at Bi'r Al-Mauna, on the road between Makka and Madina, Muhammad's companion Haram ibn Milhan was given the responsibility of taking Muhammad's letter to the chief of the Amir ibn Sa'sa' tribe. Meanwhile, upon receiving news that Amir ibn Malik had died, Haram ibn Milhan gave the letter to the former’s nephew Amir ibn Tufayl and invited those around him to Islam. Just as longtime, avowed enemy of Muhammad and the Muslims, Amir ibn Tufayl, had the envoy killed, he incited members of the tribe to launch an attack on the Muslims at Bi'r Al-Mauna. However, the people did not respond positively to this proposal due to the fact that Amir ibn Malik had guaranteed the safety of members of the delegation. Amir ibn Tufayl then appealed to certain branches of the Banu Sulaym tribe with whom they had bonds of friendship. With his provocation, armed groups from the neighboring tribes attacked the Muslims waiting at Bi'r Al-Mauna and completely unaware of any of these developments. They killed everyone except Ka’b ibn Zayd al-Najjar, who was severely wounded and left for dead, and Mundhir ibn Muhammad (or Harith ibn Thimma), who had taken the camels out for pasture at the time, and ‘Amr ibn Umayya. Unable to bear what had happened to his friends, Mundhir ibn Muhammad attacked the polytheists and was also killed. When ‘Amr ibn Umayya, who was taken as prisoner, said that he was from the tribe of Mudhar, he was released by Amir ibn Tufayl to fulfil his mother’s votive offering of emancipating a slave.

Learning of this horrific incident by means of revelation and informing his companions therein, Muhammad felt more pain and sorrow than ever before and cursed those responsible for this incident during every morning prayer for thirty or forty days on end. He sent a force of 24 men under the command of Shuja’ ibn Wahb, with the purpose of punishing the Amir ibn Sa'sa' tribe who were responsible for the Bi'r Al-Mauna massacre (8 Rabi` al-Awwal/May 629). Many animals were taken as booty and women captured in a sudden night raid. Some time later, a Muslim delegation from the Banu Amir ibn Sa'sa' came to Muhammad and requested that the women taken captive be released. Consulting with Shuja’ ibn Wahb and his companions, Muhammad released the women upon their acceptance of Islam.

Orientalist William Montgomery Watt wrote that the motive of the Banu Lahyan for attacking Muslims, was that the Banu Lahyan wanted to get revenge for the assassination of their chief at Muhammad's instigation.