Al-Qasim ibn al-Ḥasan (Sha'ban 7, 47 AH / October 4, 667 CE in Medina – Muharram 10, 61 AH / October 10, 680 CE in Karbala) was the son of Hasan ibn Ali. He supported his uncle Husayn ibn Ali in fighting off the Umayyad forces during the Battle of Karbala where he was killed at the age of 13.

Qasim was born the son of Hasan ibn Ali, the second Imam, and his wife Umm Farwa. His father was the elder son of Ali ibn Abi Talib and Fatima. Qasim was thus a great-grandson of the Islamic prophet Muhammad, who was the father of Fatima.

Hasan ibn Ali, Qasim's father died on the 28th Safar 50 AH (2 April 670 CE) by Mu'awiya I due to poisoning.

Qasim was only three years old when his father died. However he grew up in the company of his paternal family. Like his cousins, Awn and Muhammad, the sons of his paternal aunt Zaynab bint Ali, Qasim was taught fencing by his uncle Abbas ibn Ali and cousin Ali al-Akbar ibn Husayn. When Hasan's brother Husayn ibn Ali prepared to leave Medina in 680, Qasim's mother Umm Farwa asked Husayn to take her and the young Qasim with him.

Qasim asked his uncle, "Will I also be among the martyrs?" Husayn ibn Ali replied: "How do you see death?" Qasim said, "O uncle, death to me is sweeter than honey."

On the day of Ashura, Qasim, like other cousins before him, went to his uncle to ask for permission. Husayn would not permit him to fight because he was too young and Husayn could not bear the thought of anything happening to him. Qasim asked many times before going to his mother. When his mother saw that her son was upset, she gave him a letter that Qasim's father had written before he died. The letter stated:

My son Qasim, a day will come when my brother Husayn will be facing an enemy army of tens of thousands. That will be the day when Islam will need to be saved by sacrifice. You must represent me on that day.

Qasim read the letter and gave to his uncle. After reading the letter Husayn said,

O my brother's son, how can I stop you from doing what your father wanted you to do? In the Name of Allah, go! Allah be with you!"

He went to the battlefield and killed many fighters. A man came from behind and struck Qasim on the head. He fell to the ground, bleeding heavily. He called out for his uncle. Husayn and Abbas rushed over. The army of Yazid got scared thinking they were coming to attack them, creating confusion. Horses began running from one side to another, trampling and killing Qasim.^[11] Husayn and Abbas called out for him, but no reply came. When they found his body Husayn as took off his abaa and with Abbas collected his remains.

On 10 October 680 (Muharram 10, 61 AH), the day of Ashura, Qasim ibn Hasan was killed near the Euphrates, where Husayn and his followers were not allowed to get water. All the 72 companions of Husayn, including Qasim ibn Hasan were buried in a mass grave. The mass grave is located at the foot of Husayn's grave.