Castell Aberlleiniog (Castle of the Mouth of the Lleiniog) is a motte and bailey fortress near the Welsh village of Llangoed on the Isle of Anglesey, built between 1080 and 1099 by Hugh d'Avranches, 1st Earl of Chester.

It is about two miles distant from Beaumaris Castle, and was built atop a very steep hill.

There are few historical documents that detail the events the castle has seen through its life, however some do survive, including a record of a siege in 1094 by Gruffudd ap Cynan on his return from Ireland.

Gruffudd was later successful in driving Hugh out, and after this, Anglesey remained under the control of the Welsh until the arrival of Edward I two hundred years later.

The keep had broad ramparts fronted by a narrow wall rising to a parapet; each corner contained a round tower, the remains of which can still be seen.