Nguyễn Lữ (chữ Hán: 阮侶; died 1787), also known by the title of Đông Định vương (東定王, "king of Eastern Conquering"[note 1]), was the one of the Tây Sơn brothers who formed short-lived Tây Sơn dynasty of Vietnam.

Lữ had two brothers: Emperor Thái Đức, common name Nguyễn Nhạc; and Emperor Quang Trung, common name Nguyễn Huệ.

A civil war broke out between Nguyễn Nhạc and Nguyễn Huệ in 1787, the main forces withdrew from Gia Định, and marched north to support Nhạc.

Nguyễn Lữ was afraid, and retreated to Biên Hòa, leaving Phạm Văn Tham to defend Saigon.

After receiving the letter, Tham marched to Biên Hòa, with a white flag in front of the army, to justify his wrongful treatment.