Josiah Kirby Lilly Sr. (November 18, 1861 – February 8, 1948), nicknamed "J. K.", was an American businessman, pharmaceutical chemist, and philanthropist who served as president and chairman of the board of Eli Lilly and Company, the pharmaceutical firm founded by his father, Colonel Eli Lilly, in 1876. Josiah, the colonel's sole heir, began working for the company at age 14.
He graduated from Philadelphia College of Pharmacy and became superintendent of Lilly's laboratories in 1882. He succeeded his father as company president in 1898. Under his leadership, Eli Lilly and Company introduced standardized manufacturing processes, expanded its sales force, and increased research efforts to develop new drugs. The company grew into one of the largest and most influential pharmaceutical corporations in the world and the largest corporation in Indiana. Lilly's eldest son, Eli Jr., succeeded him as president in 1932. Lilly continued as chairman of the board until his death in 1948. His younger son, Josiah Jr. ("Joe"), succeeded Eli as company president in 1948.
Lilly also supported numerous charitable and civic organizations in Indianapolis and throughout Indiana. In 1937, Lilly and his two sons established the Lilly Endowment with Eli Lilly and Company stock valued at $280,000. The endowment later became one of the world's largest charitable foundations and continues the Lilly family's philanthropic legacy. Lilly also amassed a significant collection of composer Stephen Foster's music and memorabilia, which he donated to the University of Pittsburgh in 1937.