William Longshaw Jr. (April 26, 1839 – January 15, 1865) was a physician who volunteered for service in the United States Union Navy during the American Civil War. Longshaw obtained medical and pharmacy training in Boston, New York City, and New Orleans, receiving his medical degree from the University of Michigan. He was appointed acting assistant surgeon by the navy in 1862, serving aboard USS Yankee, Passaic, Penobscot, Lehigh, and finally his squadron's flagship Minnesota.