Homer Winthrop Hillyer (January 26, 1859 – January 3, 1949) was an American chemistry professor, chemical engineer, inventor, and horticulturist. He taught at the University of Wisconsin (UW) for twenty years before moving to Buffalo, New York, New York City, and Farmington, Connecticut, where he worked in the chemical industry until his retirement in 1925. He developed fungicides using atomic sulfur for fruit after obtaining a patent for agricultural products using suspended sulfur.

Hillyer was born in Waupun, Wisconsin, to industrialist and politician Edwin Hillyer and Angeline , a descendant of American colonial Robert Coe. His cousin Henry Waldo Coe was a physician and politician. His nephew Arthur Hillyer Ford was an electrical engineer and professor at Iowa State University.

Hillyer married Harriet Robbins in 1887. Robbins's father was a lobbyist for the automotive industry in Baltimore. They had no children. They helped found the Farmington Poetry Club.

Education and career Hillyer attended Ripon College before finishing his bachelor's degree in chemistry at UW in 1882. He was awarded a scholarship, followed by a graduate fellowship, at Johns Hopkins University where his research focused on aluminum alcoholates, phenoxy-ozone compounds, and phenyl ether derivatives. He was awarded his PhD in chemistry in 1885.

After receiving his PhD, Hillyer was a chemistry instructor at UW. In 1889, he was offered a professorship there to teach organic chemistry. He was one of the earliest educators in UW's school of pharmacy. He stayed there until 1905, when he decided to pursue his interest in research into pomology, in particular, growing fruit trees. A colleague suggested that Connecticut was a better place to grow apples than California, so he and his wife moved to Farmington, Connecticut.

Hillyer took a research position at General Chemical Company, where he worked on fungicide research until 1917, which took him to New York City part time. He then worked at their successor, National Aniline & Chemical Co. in Buffalo, New York, until his retirement, at which time he and his wife returned to Farmington. He died in hospice care in 1949.

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