The Cehennemağzı Caves (Turkish: Cehennemağzı Mağaraları) are a cave system of three caves lined side by side, located in the Karadeniz Ereğli district of Zonguldak Province, northern Turkey. One of the caves, which was used as a church in early Christianity, is a show cave.

The cave system is located in the Uzunmehmet neighborhood of Karadeniz Ereğli, the region known in the Antiquity as the Acheron Valley. The caves are an archaeological site affiliated with the Ereğli Museum. There are three caves lined up side by side.

The first cave is arranged in two sections. In the first section, the floor is paved with a mosaic of original plants and geometric motifs. There are columns, capitals and oil lamp niches. A small apse was opened on the eastern wall of the second section, and there are steps in front of it. This cave, which is a very old Christian church, was used as a secret place of worship in the first years of Christianity.

The second cave is located on the about 10–12 m (33–39 ft) high slope by the roadside. It is called the Koca Yusuf Cave by the local people. The cave, which is reached by a 3-step vertical staircase through a narrow entrance on the slope, continues into the mountain for 1.5 km (0.93 mi). Since a rock falling from the ceiling closed the road in the 1960s, it can only be reached to a depth of 350 m (1,150 ft). This cave, which is considered to be dug out by humans as seen from the traces of stonemasonry work, covers an area of about 400 m (4,300 sq ft), and is supported by two massive pillars.

The third cave is the largest in terms of area. Its floor is covered with groundwater. The manmade cave served as a water source cistern for the first and second caves.

According to the Greek mythology, the Cehennemağzı Mağaraları (for "Hell's Mouth Caves") is the place, where the divine hero Herakles landed in the Land of the Dead to kidnap Cerberus, the hound of Hades, the god of the dead and the king of the underworld. These caves are known to be one of the two most important prophecy centers of antiquity. The other is in the Greek city of Delphi.

The caves were opened to the public as a show cave in 2001. Also a religious tourism destination, the cave is open for visiting between 9–19 hours local time. It is closed on Mondays and weekends. It was reported that a total of 26,853 tourists visited the caves in 2016. In the first six months of 2018, the caves attracted about 13,000 domestic and foreign visitors. The number of visitors in the last five years totaled around 140,000.