Allt y Wern is a Site of Special Scientific Interest (SSSI) in the community of Llangathen in Carmarthenshire, Wales.

[1] The site was designated in 1973 and comprises 6 hectares (15 acres) of woodland of oak (Quercus robur) many of which are at least 200 years old, ash (Fraxinus excelsior) and hazel (Corylus avellana), with species-rich shrub and ground layers.

16 operations are currently listed and include activities relating to farming, and the introduction or removal of flora and fauna.

[3] Excessive grazing by domestic animals and deer reduces the natural regeneration of the wood and fencing should be improved.

Alien tree species such as sycamore and beech are present, in small numbers, but they generate excessive shade and persistent leaf litter.