The Battle of Achelous or Acheloos, also known as the Battle of Anchialus, takes place on August20, 917, on the Achelous river near the Bulgarian Black Sea coast, close to the fortress Tuthom (modern Pomorie) between Bulgarian and Byzantine forces.

The Bulgarians obtain a decisive victory that not only secures the previous successes of Simeon I but makes him de facto a ruler of the whole Balkan Peninsula excluding the well-protected Byzantine capital Constantinople and the Peloponnese.The battle is one of the worst disasters to ever befall a Byzantine army, and conversely one of the greatest military successes of Bulgaria.

Among the most significant consequences is the official recognition of the Imperial title of the Bulgarian monarchs, and the consequent affirmation of Bulgarian equality vis-à-vis Byzantium.