John R. Callahan (1853 – February 12, 1918) was a pioneer in the field of dentistry and particularly dental research in the late 19th and early 20th centuries.

He also contributed papers on materials and methods for filling root canals; notably, chloro-percha.

His original contribution of the use of sulfuric acid for opening root canals was significant at the time as were his investigations on dental materials and patient management problems.

First, a bronze bust of Callahan sculpted by Frederick C. Hibbard of Chicago was placed on the grounds of the Cincinnati General Hospital – in 1937 it was moved to the College of Dentistry of the Ohio State University.

The names of the recipients are inscribed on a bronze plaque displayed in the Ohio Dental Association headquarters.