Saint Erth is bounded on the north by Phillack, on the east by Gwinear and Crowan, on the south by Breage and St Hilary, and on the west by Ludgvan and Lelant. Little is known about St Ercius from whom this village gets its name.

The river Hayle has its source in the parish of Crowan. The parish of St Erth stretches about three miles south from the Hayle estuary and has had a bridge over the river Hayle for at least six hundred years.

The railway station was formerly known as the St Ives Road Station and in 1877 this section of railway had the last section of broad gauge track ever laid.