In J. R. R. Tolkien's legendarium, Nargothrond ("The great underground fortress on the river Narog"), called Nulukkhizdīn by the Dwarves, is the stronghold built by Finrod Felagund, delved into the banks of the river Narog in Beleriand, and the lands to the north (the Talath Dirnen or Guarded Plain) ruled by the city.
Inspired by Menegroth in Doriath, and seeking a hidden place from which to be safe from the forces of Morgoth, Finrod established it in the early years of the First Age, in the Caverns of Narog beneath the forested hills of Taur-en-Faroth on the western bank of Narog. The original denizens of this huge cave system had been the Noegyth Nibin, the so-called 'Petty-dwarves', but whether they were driven out of their homes by Finrod's people, or earlier by the nearby Sindar, is not known.
Other Dwarves, stonemasons from the Ered Luin, then helped the Noldor enlarge and landscape the caves into a formidable and concealed stronghold. Originally, it could only be reached by a narrow path along the banks of the river, but later a bridge was built across Narog.
Finrod ruled Nargothrond until he joined Beren in his quest for the Silmaril, and the regency passed to his nephew (or brother) Orodreth. However, two of their cousins, Celegorm and Curufin also lived in Nargothrond at this time and commanded a more powerful faction than Orodreth, until their homicidal dispositions were exposed following Finrod's death, at which point they were exiled.
Túrin Turambar later came to Nargothrond and became one of its greatest warriors, but he also persuaded the people to fight openly against Morgoth (the bridge was built at this time), which eventually led to its sack by the army of the dragon Glaurung.
Glaurung then used Nargothrond as his lair until his death not long afterwards at Túrin's hands, after which the caves were claimed by Mîm, the last of the Petty-dwarves, until he himself was slain by Húrin, Túrin's father. After Húrin's deed, the caves were probably completely abandoned, as they fall out of recorded history, but they were certainly drowned and lost along with the rest of Beleriand at the end of the First Age.