We have less information about Syros during this period.

During the Persian war, in the 5th century B.C., the Cyclades sided with Athens. Syros also was a member of the Athenian Alliance and paid an annual tax of 1.500 drachmas.

The island's position near to the holy island of Delos, combined with its commercial development, led to its economic recovery at the end of the 5th century B.C. Proof of this is an increase of taxes. Great activity of the port of Syros is evident. Coins from Syros were circulated as far as Syria. The centre of all this activity was situated on the site of present Hermoupolis.

Syros was an independent island governed in a way similar to that of Athens, with its own parliament, mint and court.