Deimachus (; Greek Δηίμαχος) was a Greek of the Seleucid Empire who lived during the third-century BCE. He became an ambassador to the court of Bindusara "Amitragata" (son of Chandragupta Maurya) in Pataliputra in India.[1]

As an ambassador, he was the successor to the famous ambassador and historian Megasthenes. Both of them were mentioned by Strabo:

Deimachus apparently wrote extensively on India, and is quoted as a reference in geographical matters, although his works are now lost:

"It is likely there are more than 3000 stadia, but taking this number, if we add thereto the 30,000 stadia, which Deimachus states there are between [the southern extremity of India] and the country of the Bactrians and Sogdians, we shall find both of these nations lie beyond the temperate zone and habitable earth." (Strabo II,1,14)

Strabo, however, disputed these figures, and some of the fanciful accounts of both men, although they also brought extensive knowledge about India: