Deimachus or Daimachus (Template:IPAc-en; Template:Lang-grc) was a Greek from Plataeae, who lived during the third-century BCE. He became an ambassador to the court of the Mauryan ruler Bindusara "Amitragatha" (son of Chandragupta Maurya) in Pataliputra in India.Template:Sfn
Deimachus was sent by Antiochus I Soter.
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.
Strabo, however, disputed these figures, and some of the fanciful accounts of both men, although they also brought extensive knowledge about India.