Kenneth Vivian Thimann (August 5, 1904 – January 15, 1997) was an English-American plant physiologist and microbiologist known for his studies of plant hormones, which were widely influential in agriculture and horticulture. He isolated and determined the structure of auxin, the first known plant hormone. He spent most of his early career (1935–1965) at Harvard University, and his later career (1965 - ) at the University of California, Santa Cruz.