Ichthyology is the branch of zoology devoted to the study of fish, including bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). According to FishBase, 35,800 species of fish had been described as of March 2025.

The word is derived from the Ancient Greek words ἰχθύς, ikhthus, meaning "fish"; and λόγος, logos, meaning "study".

The study of fish dates from the Upper Paleolithic Revolution (with the advent of "high culture"). The science of ichthyology was developed in several interconnecting epochs, each with various significant advancements.

The study of fish receives its origins from the human desire to feed, clothe, and equip themselves with useful implements. According to Michael Barton, a prominent ichthyologist and professor at Centre College, "the earliest ichthyologists were hunters and gatherers who had learned how to obtain the most useful fish, where to obtain them in abundance, and at what times they might be the most available". Early cultures expressed these insights through abstract yet recognizable forms of artistic expression.

Informal, scientific descriptions of fish are represented within the Judeo-Christian tradition. The Old Testament laws of kashrut forbade the consumption of fish without scales or appendages.^{[citation needed]} Theologians and ichthyologists believe that the apostle Peter and his contemporaries harvested the fish that are today sold in modern industry along the Sea of Galilee, presently known as Lake Kinneret. These fish include cyprinids of the genera Barbus and Mirogrex, cichlids of the genus Sarotherodon, and Mugil cephalus of the family Mugilidae.

Aristotle incorporated ichthyology into formal scientific study. Between 333 and 322 BC, he provided the earliest taxonomic classification of fish, accurately describing 117 species of Mediterranean fish. Furthermore, Aristotle documented anatomical and behavioral differences between fish and marine mammals. After his death, some of his pupils continued his ichthyological research. Theophrastus, for example, composed a treatise on amphibious fish. The Romans, although less devoted to science, wrote extensively about fish. Pliny the Elder, a notable Roman naturalist, compiled the ichthyological works of indigenous Greeks, including verifiable and ambiguous peculiarities such as the sawfish and mermaid, respectively. Pliny's documentation was the last significant contribution to ichthyology until the European Renaissance.

The writings of three 16th-century scholars, Hippolito Salviani, Pierre Belon, and Guillaume Rondelet, signify the conception of modern ichthyology. The investigations of these individuals were based upon actual research in comparison to ancient recitations. This property popularized and emphasized these discoveries. Despite their prominence, Rondelet's De Piscibus Marinis is regarded as the most influential, identifying 244 species of fish.

The incremental alterations in navigation and shipbuilding throughout the Renaissance marked the commencement of a new epoch in ichthyology. The Renaissance culminated with the era of exploration and colonization, and upon the cosmopolitan interest in navigation came the specialization in naturalism. Georg Marcgrave of Saxony composed the Naturalis Brasilae in 1648. This document contained a description of 100 species of fish indigenous to the Brazilian coastline. In 1686, John Ray and Francis Willughby collaboratively published Historia Piscium, a scientific manuscript containing 420 species of fish, 178 of these newly discovered. The fish contained within this informative literature were arranged in a provisional system of classification.