Baba Baj Singh (died 9 June 1716; his first name is alternatively spelt as Baaj), also known as Baj Bahadur, was a Sikh general, governor, scholar and martyr from present-day India. Baj Singh was the cousin of Bhai Mani Singh.
Baj Singh's family was native to Mirpur Patti, a village in Amritsar district of the Punjab. He was part of a Panj Piare quintet commanded, by Guru Gobind Singh, to accompany Banda Singh Bahadur and provide him counsel on his northwards conquest of Sirhind.
He was executed on 9 June 1716 on the outskirts of Delhi, on the bank of the Yamuna river along with his seven brothers and Banda Singh Bahadur.