An Tư (chữ Hán: 安姿) was a princess during the Trần Dynasty (1225–1400), the youngest offspring of emperor Trần Thái Tông. Today, nobody knows exactly her birth and death year. Đại Việt sử kí toàn thư only wrote “ordering servants to give An Tư to Toghan as a present was to decrease disaster happened to our country”.
In early 1285, an army of the Yuan Dynasty of China invaded Đại Việt for the second time. Toghan led his army to attack Gia Lâm, and surrounded the Vietnamese capital, Thăng Long. Grand emperor Trần Thánh Tông and emperor Trần Nhân Tông went to Tam Trĩ by means of a small boat, while the official royal boat went to Ngọc Sơn to mislead the invaders, but they were discovered. On March 9, Yuan marine troops surrounded Tam Trĩ and almost captured the Grand emperor and emperor. Needing time to consolidate and organize the army and slow down the pace of the Yuan army, Trần Thánh Tông unavoidably gave his little sister, princess An Tư, to Toghan, as a gift.
After that, in April, the Trần army began to counterattack, having win after win. The Yuan army completely lost. Toghan had to hide in a bronze pot to cross the border. Dai Viet history did not mention An Tư at all, but in An Nam Chí lược, written by Lê Tắc, Trần Kiện’s servant, following his master to China, wrote, "Crown prince (mention Toghan) married a Trần woman, had two children." Perhaps the Trần woman here was An Tư.
The novel Princess An Tư, written by Nguyễn Huy Tưởng, was based on this true story.