Walter Sydney Lazarus-Barlow FRCP (born Walter Sydney Lazarus; 18 July 1865 – 15 January 1950) was an English physician who was professor of experimental pathology at the Middlesex Hospital. He was a specialist in cancer and was one of the first physicians to research the effects of X-rays and radium on that disease. In 1909, he gave the Croonian Lecture on the subject of Radioactivity and Carcinoma.