Louis J. Ignarro (born May 31, 1941) is an American pharmacologist. For demonstrating the signaling properties of nitric oxide, he was co-recipient of the 1998 Nobel Prize in Physiology or Medicine with Robert F. Furchgott and Ferid Murad.

Currently, he is professor of pharmacology at the UCLA School of Medicine's department of molecular and medical pharmacology in Los Angeles, which he joined in 1985. Before relocating to California, he was a professor of pharmacology at Tulane University School of Medicine, New Orleans, for 12 years. Ignarro has also previously worked as a staff scientist, research department, for the pharmaceutical division of CIBA-GEIGY Corporation in New York.

Ignarro has published numerous research articles. He received the Basic Research Prize of the American Heart Association in 1998. This was in recognition of his outstanding contributions to the advancement of cardiovascular science. That same year, he was inducted into the National Academy of Sciences and the following year, into the American Academy of Arts and Sciences. Because nitric oxide is indirectly involved in the action of this drug, he is sometimes referred to as the "Father of Viagra".