Y Gaer (Latin: Cicucium) is a Roman fort situated near modern-day Brecon in Mid Wales, United Kingdom.

It was part of a chain of similar forts, such as Gobannium at Abergavenny, a day's march away down the Usk valley, and larger bases, such as Moridunum (Carmarthen) via Alabum (Llandovery), Cardiff Roman Fort to the south and Isca Augusta, Caerleon, the main base for the Roman legion locally.

[1] The site was excavated in the 1920s by Sir Mortimer Wheeler, a prominent archaeologist of his day.

The fort was built for a contingent of up to 500 cavalrymen, recruited originally in Spain from the Vettones, and these Vettonian cavalry would have played a significant part in the conquest of the area held by the Silures.

Several artifacts have been removed to local museums, such as a tombstone of a young cavalryman called Candidus, held at the Y Gaer cultural hub in Brecon.