Trần Hoảng was born on September 25, 1240 (in the lunar calendar), as the second prince but the first natural son of King Trần Thái Tông and Empress Consort Thuận Thiên. He had a half-brother on the mother's side, Prince Tĩnh Quốc () Trần Quốc Khang, who was born after the forced marriage by Trần Thủ Độ between Emperor Nhân Tông and Princess Thuận Thiên. In fact, although being Thái Tông's first prince, Trần Quốc Khang was the son of his elder brother Prince Hoài Trần Liễu, therefore officially he was not chosen by Thái Tông as the heir, a position which ultimately belonged to Prince Trần Hoảng. Besides Prince Tĩnh Quốc, Trần Hoảng had other younger brothers, including Prince Chiêu Minh Trần Quang Khải (who was also born to Empress Thuận Thiên), Prince Chiêu Quốc Trần Ích Tắc, and Prince Chiêu Văn Trần Nhật Duật.