Borommarachathirat I

สมเด็จพระบรมราชาธิราชที่ 1

King Borommarachathirat I or King Borom Rachathirat I (), also known as Khunluang Pha Ngua (; r. 1370–1388), was the third king of the Ayutthaya Kingdom. He was the older brother of King U-thong's consort and the uncle of King Ramesuan.

Prior to becoming king of Ayutthaya, Borommarachathirat I was the king of Suphannaphum (vassal to Ayutthaya). He was the brother-in-law of King Uthong, and the maternal uncle of King Ramesuan. Less than a year after King Uthong's death, he forced King Ramesuan from power and seized the throne. He was a capable warrior, his reign marked Ayutthaya's expansion to the north. He suppressed a rebellion in the Sukhothai Kingdom (1371–78) and subjugated major northern centres such as Phitsanulok. When he invaded Chiang Mai, his forces were defeated and repulsed at the Battle of Sen Sanuk, near Chiang Mai.

After his death in 1388, his son Thong Lan reigned for only a week before Ramesuan, who had retreated to Lavo, returned and overthrew him. Ramesuan later reclaimed the throne for a second reign.