Jonathan Sessler (born 20 May 1956 in Urbana, Illinois) is a professor of chemistry at The University of Texas at Austin. He is notable for his pioneering work on expanded porphyrins and their applications to biology and medicine. He is a co-founder of Pharmacyclics, Inc., (NASDAQ: PCYC) a company that works with expanded porphyrins, and Anionics, Inc., which develops anion recognition chemistry.
Sessler received his Bachelor of Science in Chemistry in 1977 from the University of California, Berkeley and his Ph.D. in chemistry in 1982 from Stanford University. He continued as a post-doctoral fellow at L'Université Louis Pasteur, and worked in Kyoto, Japan before becoming an Assistant Professor of Chemistry at The University of Texas at Austin in 1984.
He was treated for cancer at Stanford University Medical Center during work on his Ph.D. Discussions with his colleagues regarding possible treatments eventually led to the discovery of so-called "texaphyrin" molecules, named as such due a semblance to the shape of the five-pointed star in the State flag of Texas., In 1996, Sessler described possible uses of these compounds in medicine in the Proceedings of the National Academy of Sciences journal. His company, Pharmacyclics, Inc., which went public in 1995, licensed the technology behind "texaphyrins" from the University to develop commercial and medical uses for the molecules.
Sessler has written extensively, authoring or co-authoring over 350 publications. In addition, he has co-authored two books and edited a third. He holds more than 70 U.S. patents. Dr. Sessler has received many awards and much recognition, as a Sloan Fellow (1999), Fellow of the AAAS (1999), Dreyfus Teacher-Scholar (1999), ACS Cope Scholar (1999), recipient of the JSPS and Alexander von Humboldt Senior Fellowships (1999), and winner of the Izatt-Christensen Award (1999).
Sessler has participated in hosting chemistry circuses on the UT campus.,  These circuses are held for different grade levels from elementary to college, and aim to expose students to "fun" aspects of chemistry. Sessler has also lectured on how the disease Porphyria, caused by a deficiency in porphyrins, may be related to the origin of myths about vampires and werewolves.
- Jonathan L. Sessler — Department of Chemistry & Biochemistry, The University of Texas at Austin. Accessed 14 March 2006.
- Company History & Overview — Phaymacyclics.com. Accessed 14 March 2006.
- Jonathan Sessler Distinguished Lecture Series Acknowledgement — Stanford University. Accessed 14 March 2006.
- "Interview: Discovering the Texas molecule" - - Jonathan Sessler talks to The Royal Society of Chemistry about what motivated him to discover texaphyrin and about the aims of the companies he co-founded