Putnam County is a county located in the northwestern part of the U.S. state of Ohio. As of the 2020 census, the population was 34,451. Its county seat is Ottawa. The county was created in 1820 and later organized in 1834. Its name is in honor of Israel Putnam, a colonial officer in the French and Indian War, who served as a general in the American Revolutionary War. The Blanchard River, which passes through the county, was a key transportation route for early European-American settlers.

Israel Putnam, popularly known as "Old Put", was an American military officer and landowner who fought with distinction at the Battle of Bunker Hill during the American Revolutionary War (1775–1783). He also served as an officer with Rogers' Rangers during the French and Indian War (1754–1763), when he was captured by Mohawk warriors. He was saved from the ritual burning given to enemies by the intervention of French officer Molang, with whom the Mohawks were allied. Putnam's courage and fighting spirit became known far beyond his home of Connecticut's borders through the circulation of folk legends in the American colonies and states celebrating his exploits.