Harold Joel Conn (May 29, 1886 – November 10, 1975) was an American agricultural bacteriologist, known for his work on soil microbiology and bacterial staining techniques. He was one of the founders of the Biological Stain Commission and also founded their journal, Stain Technology (now Biotechnic & Histochemistry). He served as President of the Society of American Bacteriologists (now the American Society for Microbiology) in 1948.