Porth yr Ogof is a solutional cave near the village of Ystradfellte, near the southern boundary of the Brecon Beacons National Park in Wales.

Porth yr Ogof is most often entered through the wide main entrance, either using a dry ledge on the left or by wading through the knee-deep river to the centre and right.

Challenging elements of the cave interior include the "Wormhole" (a curving crawl tube on the right wall of the main entrance), the "Letterbox" (a rectangular space in the passage) and the "Creek", further inside the cave.

White Horse Pool, named after the shape of the calcite deposit on the back wall, is several metres in depth despite the shallowness of the edges, and there are many sumps (completely flooded passageways), the majority of which exist in the portions of the cave north of the Tradesman's Entrance.

[4] The cave lies in the valley of the Afon Mellte and is located in a comparatively narrow band of Carboniferous Limestone.