Trwyn Du Lighthouse is a lighthouse between Dinmor Point near Penmon and Ynys Seriol, or Puffin Island, at the eastern extremity of Anglesey, marking the passage between the two islands.
The first lighthouse was erected in 1838,. The present Lighthouse is 29m tall and was designed by James Walker and built in 1835-1838. It was his first sea-washed tower, and a prototype for his more ambitious tower on the Smalls. The Lighthouse has a stepped base designed to discourage the huge upsurge of waves that had afflicted earlier lighthouses on the site and reduce the force of the water at the bottom of the tower. Austere vertical walls, a tower with crenellated stone parapet, in preference to iron railings on the gallery, and which narrows in diameter above the half-way point, are a features used by Walker in his other lighthouse designs. The tower is distinguished by its original three black bands painted on a white background.
Walker also pioneered, unsuccessfully, the use of a primitive water closet, comprising a specially designed drain exiting at the base of the tower. The stepped design of the lighthouse may have helped water exit the closet, but surges of seawater made its use difficult during heavy weather!