Marcus Ward Lyon, Jr. (February 5, 1875 – May 19, 1942) was an American mammalogist, bacteriologist, and pathologist. He was born into a military family, and demonstrated an early interest in zoology by collecting local wildlife around his father's army posts. He graduated from Brown University in 1897, and continued his studies at George Washington University while working part-time at the United States National Museum (USNM).