The Roman Catholic Diocese of Ajmer (Latin: Aimeren(sis)) is a suffragan Latin diocese in the ecclesiastical province of Agra in northwestern India, yet still depends on the missionary Roman Congregation for the Evangelization of Peoples.

As per 2014, it pastorally served 9,980 Catholics (0.0% of 22,073,000 total) on 146,690 km² in 10 parishes and 25 missions with 60 priests (40 diocesan, 20 religious), 479 lay religious (27 brothers, 452 sisters) and 18 seminarians.

The Diocese of Ajmer-Jaipur comprised twenty four districts, namely Ajmer, Alwar, Baran, Barmer, Bikaner, Bundi, Churu, Dausa, Hanumangarh, Jaipur, Jaisalmer, Jalore, Jhalawar, Jhunjhunu, Jodhpur, Karauli, Kota, Nagaur, Pali, Sawai Madhopur, Sikar, Sirohi, Sri Ganganagar and Tonk in the state of Rajasthan.

In July, 1890 the Holy See detached a very large portion of the Archdiocese of Agra to form a new unit, the Rajasthan mission entrusting it to the Capuchin Fathers of the Paris Province who were to work under the supervision of the Archbishop.

In December, 1891, the new mission became the Prefecture Apostolic of Rajputana and in 1913, Caumont was appointed the first bishop of Ajmer.