Leucippus (; , Leukippos; fl. 5th century BCE) is reported in some ancient sources to have been a philosopher who was the earliest Greek to develop the theory of atomism — the idea that everything is composed entirely of various imperishable, indivisible elements called atoms. The name Leucippus is often associated as master to pupil with that of Democritus, the philosopher who was also touted as the originator of the atomic theory. However, a brief notice in Diogenes Laertius’s life of Epicurus says that on the testimony of Epicurus, Leucippus never existed. As the philosophical heir of Democritus, Epicurus's word has some weight, and indeed a controversy over this matter raged in German scholarship for many years at the close of the 19th century. Furthermore, in his Corpus Democriteum, Thrasyllus of Alexandria, an astrologer and writer living under the emperor Tiberius (14–37 CE) compiled a list of writings on atomism that he attributed to Democritus to the exclusion of Leucippus. The present consensus among the world's historians of philosophy is that this Leucippus is historical. The matter must remain moot unless more information is forthcoming from the record.

Leucippus (; , Leukippos; fl. 5th century BCE) is reported in some ancient sources to have been a philosopher who was the earliest Greek to develop the theory of atomism — the idea that everything is composed entirely of various imperishable, indivisible elements called atoms. The name Leucippus is often associated as master to pupil with that of Democritus, the philosopher who was also touted as the originator of the atomic theory. However, a brief notice in Diogenes Laertius’s life of Epicurus says that on the testimony of Epicurus, Leucippus never existed. As the philosophical heir of Democritus, Epicurus's word has some weight, and indeed a controversy over this matter raged in German scholarship for many years at the close of the 19th century. Furthermore, in his Corpus Democriteum, Thrasyllus of Alexandria, an astrologer and writer living under the emperor Tiberius (14–37 CE) compiled a list of writings on atomism that he attributed to Democritus to the exclusion of Leucippus. The present consensus among the world's historians of philosophy is that this Leucippus is historical. The matter must remain moot unless more information is forthcoming from the record.