Rani Datar Kaur (died 1813), the daughter of Sardar Ran Singh Nakai, the third ruler of Nakai Misl of Baherwal, was the 2nd wife of Maharaja Ranjit Singh. Originally named Raj Kaur, she changed her name to Datar Kaur as Raj Kaur was also the name of Ranjit Singh's mother. She was married to the Maharaja in 1798 who lovingly addressed her as Mai Nakain. In 1802, she gave birth to Kharak Singh, the heir apparent of Ranjit Singh. She took an active interest in the affairs of the State and accompanied her son when he was sent out on an expedition to Multan (30°11'N 71°29'E) in 1818. She died on 20 June 1838. Her grandson was Maharaja Nau Nihal Singh (1839–40).
Bibi Raj Kaur was the youngest of four siblings. She had three brothers; Bhagwan Singh, Gyan Singh and Khazan Singh. Their father, Sardar Ran Singh Nakai, the third ruler of Nakai Misl, had fought repeatedly against Kamar Singh, the ruler of Sayyadwala. Sometime before his death in 1781, he defeated him and captured Sayyadwala. Sardar Ran Singh had greatly expanded his Misl's strength so it became dominant among its neighbours. At its high point under Ran Singh it ruled Kasur, Sharakpur, Gugera pargana, and the Kharral fort of Kot Kumaliah. His son, Bhagwan Singh, succeeded him but could not hold his territory against Wazir Singh, the brother of Kamar Singh who retook Sayyadwala. Realizing he might lose all of his territory, around 1784, Bhagwan Singh set up the engagement of his sister, Raj Kaur, to Ranjit Singh, who was the son of Mahan Singh, the leader of the powerful Sukerchakkia Misl, in order to gain a powerful ally.
Bhagwan Singh and Wazir Singh continued to engage in constant warfare and Bhagwan Singh was killed in battle. In 1789, Gyan Singh, the second son of Sardar Ran Singh Nakai, succeeded his brother, and in 1798, married his sister Raj Kaur to Ranjit Singh. Raj Kaur also being the name of a mother of Maharaja Ranjit Singh, she took the name of Datar Kaur. In some historical accounts, mistakenly, marriage of Ranjit Singh has been mentioned with two daughters of Nakai Misl i.e. Raj Kaur and Datar Kaur. However, she was one and the same person.
In 1802, she gave birth to Prince Kharak Singh, the heir apparent of Sikh Empire. Even though Ranjit Singh had many marriages, Datar Kaur remained his favourite and he fondly called her Mai Nakain. In 1807, Gyan Singh died and his son Sardar Kahan Singh Nakai succeeded him.
Even though his favourite Rani was from the house of the Nakais, the relationship between Sukerchakkias and Nakais remained an uneasy one. After Ranjit Singh had declared himself a Maharaja in 1801 after consolidating majority of the Misls, this ambitious chief had been eyeing the Nakai territory. He spared it till the death of Gyan Singh in 1807, but soon after suggested Kahan Singh to join the Darbar at Lahore, which the proud Nakai chief steadily refused. Finally, in 1810, Ranjit Singh sacked Kahan Singh and annexed all the Nakai territories. Even the Rani could not prevent it.