The Essex emerald (Thetidia smaragdaria) is a moth of the family Geometridae.

The species was first described by Johan Christian Fabricius in 1787 as Phalaena smaragdaria.

[1] It is distributed throughout the Palearctic region with records from many European countries.

The British subspecies Thetidia smaragdaria maritima was last seen in 1991 in Kent and is now presumed extinct.

The larvae feed on Artemisia maritima and Achillea millefolium.