Agios Konstantinos is situated in the hills in the southeastern part of the Attica peninsula, at about 160 m elevation. It is 4 km west of the Aegean Sea coast at Lavrio and 38 km southeast of Athens city centre. The municipal unit has an area of 10.450 km2. There are several mines in the vicinity of Agios Konstantinos and neighboring Lavrio. In antiquity it was known for silver mines, currently minerals including azurite, chalcoalumite, calcite, austinite and adamite are found.
The community Agios Konstantinos also includes the village Esperídes (pop. 69).