Al-Qasim ibn Hasan (Arabic: القاسم بن الحسن) (c. 666 – 10 October 680) was the son of the Shī`a Imām, Hasan ibn Ali, and grandson of Ali ibn Abi Talib. He was born in 666, three years before the death of his father.
When Hasan's brother Husayn prepared to leave Medina in 670, Qasim's mother Umm Farwa asked Husayn to take her and Al-Qasim with him. Husayn agreed. Qasim, although only 14 years old, had, like his cousins, Aun and Muhammad, who were his aunt Zaynab's sons, learnt fencing from his uncle Abbas and his cousin Ali Akbar. He took part in the battle of Karbala, in which he was martyred. Before the Battle of Karbala, on the 9th day of Muharram. Al-Qasim was married to Husayn ibn Ali's eldest daughter, Fatimah Kubra bint Hussain, who was 11 at the time. It is said that Husayn made a promise to his brother Hasan that he will marry his daughter to his son. Therefore, knowing that he was going to die, he arranged the marriage before the day of Ashura to keep his promise. On the day of Ashura, Al-Qasim, like other cousins before him, was eager to fight for his uncle whom he loved dearly, so he went to his uncle to ask for permission. Husayn would not permit him because he was so young and could not bear the thought of anything happening to him, so he did not permit him. This went on for many times until Al-Qasim gave up and went to the tent his mother was in. His mother was very upset to see her son like this so she gave him a letter that Hasan had written for his son before he died. She gave Al-Qasim the letter. In the letter it was written, "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." Al-Qasim read the letter and rushed to his uncle and gave him the letter. 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!" When he got up on the horse he looked like a shining moon. Al-Qasim was a very handsome boy. When he went to the battle field, he fought with all of his strength and killed a lot of the enemies. But, he was only a boy and was thirsty for three days due the blockage of the river by Yazid I and his army. A man came from behind and hit Al-Qasim with his sword to which he fell and cried aloud, "O, dear uncle, peace be upon you" Husayn rode out upon hearing this. The soldiers tried to stop him, when finally he came to where Al-Qasim had fallen. He saw Al-Qasim's body trampled to pieces by the horses of the soldiers who had tried to stop them from reaching to their nephew. When the time came for Husayn to take his nephew's body back to the tents he took off his aba, spread it on the ground and picked the pieces of the body up as one would collect flowers from a garden.