Deimachus of Plataea was a Hellenistic historian and diplomat of the 3rd century BCE. He served as a member of a Seleucid embassy sent by King Antiochus I Soter to the court of the Mauryan emperor Bindusara in India. Beyond this mission, no further biographical details are recorded.

His sole known work is a lost account of India, based on his experiences as an envoy. It survives only in fragments cited by later authors, principally the geographer Strabo.

Deimachus is significant as one of the earliest Greek sources on India following Alexander the Great. Alongside writers like Megasthenes, he contributed descriptions of Indian geography and customs to the Greco-Roman world. Ancient critics like Strabo, however, treated his work with skepticism, noting it blended factual observation with fable. His fragments remain important for the study of early Hellenistic ethnography and Indo-Greek contact.