Trần Thánh Tông (1240–1290), given name Trần Hoảng , was the second emperor of the Trần Dynasty, reigning over Đại Việt from 1258 to 1278. After ceding the throne to his son Trần Nhân Tông, Thánh Tông held the title Thái thượng hoàng from 1279 to his death in 1290. During the second and the third Mongol invasions of Đại Việt, the Retired Emperor Thánh Tông and the Emperor Nhân Tông were credited as the supreme commanders who led the Trần Dynasty to the final victories and as a result established a long period of peace and prosperity over the country. With his successful ruling in both military and civil matters, Trần Thánh Tông was considered as one of the greatest emperors of not only the Trần Dynasty but also the whole dynastic era in the History of Vietnam.
Thánh Tông was born on September 25 of Lunar calendar 1240 as Trần Hoảng, the second prince but the first natural son of the Emperor Trần Thái Tông and Empress Consort Thuận Thiên. He had a half-brother on the mother's side, Prince Tĩnh Quốc (Vietnamese: Tĩnh Quốc đại vương) Trần Quốc Khang, who was born after the forced marriage by Trần Thủ Độ between the Emperor Nhân Tông and Princess Thuận Thiên. In fact, although being Thái Tông's first prince, Trần Quốc Khang was son of his elder brother Prince Hoài Trần Liễu, therefore officially he was not chosen by Thái Tông as the first in line of successor, a position which ultimately belonged to prince Trần Hoảng. Besides Prince Tĩnh Quốc, Trần Hoảng had some other younger brothers including Prince Chiêu Minh Trần Quang Khải, who was also given birth by Empress Thuận Thiên, Prince Chiêu Quốc Trần Ích Tắc or Prince Chiêu Văn Trần Nhật Duật.
In 1257, Crown Prince Trần Hoảng witnessed the first Mongol invasion in Vietnam.In the beginning, Đại Việt army suffered several defeats by an overwhelming force which had already conquered a vast area in Asia. Several high-ranking officials of Trần Dynasy were so scared that Prince Khâm Thiên Trần Nhật Hiệu, younger brother of Thái Tông, even suggested the Emperor that they might escape from Đại Việt to the Song Dynasty. Thanks to the firm faith of the Emperor Thái Tông, grand chancellor Trần Thủ Độ and the talent of military generals such as Prince Hưng Đạo Trần Quốc Tuấn and Lê Phụ Trần, the Trần Dynasty was able to drive back the invasion and ultimately re-etablished the peace in Đại Việt in December 1257.
Returned to the capital Thăng Long, Trần Thái Tông decided to cede the throne to Crown Prince Trần Hoảng, now Trần Thánh Tông, on the 24th day of the second lunar month (March 30) 1258. After the coronation, Thánh Tông changed the era name to Thiệu Long (1258–1272), during his reign, the emperor had one more era name which was Bảo Phù (1273–1278). Although passing the throne to his son, Thái Tông continued to co-rule the country with Thánh Tông in the position of Retired Emperor (Thái thượng hoàng) from 1259 to his death in 1277.