Semachidae or Semachidai (Ancient Greek: Σημαχίδαι) was a deme of ancient Athens, in the district of Epacria.
The site of Semachidae is tentatively located near modern Vredou.
This article incorporates text from a publication now in the public domain: Smith, William, ed. (1854–1857). "Attica". Dictionary of Greek and Roman Geography. London: John Murray.
38°09′31″N 23°54′37″E / 38.158624°N 23.9103345°E