Sir Walter Baldwin Spencer KCMG FRS (23 June 1860 – 14 July 1929), commonly referred to as Sir Baldwin Spencer, was a British-Australian evolutionary biologist, anthropologist and ethnologist. He is known for his fieldwork with Aboriginal peoples in Central Australia, contributions to the study of ethnography, and academic collaborations with Frank Gillen. Spencer introduced the study of zoology at the University of Melbourne and held the title of Emeritus Professor until his death in 1929. He was elected a Fellow of the Royal Society in 1900 and knighted in 1916.

Spencer was born on 23 June 1860 in Stretford, Lancashire, England to Martha (née Circuit) and Rueben Spencer. He was educated at Old Trafford school and Manchester School of Art, where he received training in drawing.

In 1879, Spencer began study at Owens College (University of Manchester), where he first developed an interest in evolutionary biology. In 1884, he obtained a BA in biology from the University of Oxford, with first-class honours. Through his time at Oxford, Spencer attended lectures by anthropologist Edward Burnett Tylor, which began a lifelong interest in the field.

As a graduate, Spencer worked as a teaching assistant to his former professor Henry Nottidge Moseley, where he aided in the transferral of General Pitt-Rivers's collection of anthropological artefacts from the South Kensington Museum to Oxford University, which had built a museum to house it. He obtained a fellowship at Lincoln College, Oxford in 1886.

Having already contributed various papers to scientific journals, one of which, on the Pineal eye in lizards, had aroused much interest, and having applied for the professorship of biology at Melbourne in June 1886 was elected to that chair in January 1887.

Spencer arrived in Australia in March of that year and set about organising his new school (the chair had just been founded), successfully receiving a building grant of £8000 to develop new lecture rooms and laboratories. He showed much capability as a lecturer and organiser, and also took a full part in the general activities of the university. His interests were not confined to his university duties; he took a leading part in the proceedings of the Royal Society of Victoria, the Field Naturalists Club of Victoria, and the Australasian Association for the Advancement of Science, and did valuable work for those bodies.

In 1894 a new field was opened up for Spencer when he joined the W.A. Horn scientific expedition which left Adelaide in May 1894 to explore Australia. In July he met Francis James Gillen at Alice Springs with whom he was to be so much associated in the study of the Aboriginal peoples. The expedition covered some 2000 miles in about three months and on his return Spencer busied himself with editing the report, to which he also largely contributed. It was published in 1896. In November 1896 Spencer was again at Alice Springs beginning the work with Gillen which resulted in Native Tribes of Central Australia, published in 1899 and partly opposed by Carl Strehlow and Moritz von Leonhardi. He continued this work with Gillen during the vacations of the two following years. A large amount of material relating to tribal customs was accumulated, and the book appeared with the names of both Gillen and Spencer on the title page. D. J. Mulvaney, writing about Spencer in the Australian Dictionary of Biography, tells us that while Spencer was "a kindly humanitarian in practice, in theory he saw Aboriginal people simply as dehumanised 'survivals' from an early stage of social development" and that while records are invaluable, he nonetheless made "unacceptable value judgements" on indigenous Australians.

Spencer was recruited as science writer for the Australasian by its editor, David Watterston.