Leonard said he will support Pruitt in the November general election against Democratic attorney Jim Priest of Oklahoma City.
Leonard is the son of senior federal judge Tim Leonard. His father-in-law is former Gov. Frank Keating. He previously worked as a prosecutor in Cleveland County.
Pruitt has served as a state senator and unsuccessfully ran for Congress and lieutenant governor. He had the backing of former Arkansas Gov. Mike Huckabee, who announced his support in a Twitter posting Monday.
For the past 16 years, Drew Edmondson has been attorney general. He did not seek re-election so he could run for the Democratic nomination for governor. Oklahoma has only had one Republican attorney general since statehood — G.T. Blankenship began his term in 1967.