Joseph Breintnall (died 1746) was an influential American merchant and amateur naturalist. He was the first Secretary of the Library Company of Philadelphia and the sheriff of Philadelphia from 1735–1738. His name is usually remembered alongside that of his friend and collaborator, Benjamin Franklin. An early member of Franklin's Junto, Breintnall co-authored a series of letters with Franklin, under the name of "The Busy-Body." The 32 letters were printed serially in Andrew Bradford's newspaper The American Weekly Mercury in 1729.