In Breton folklore, the Nain (Breton for dwarf or small-dwarf) are fearsome, gargoyle-like spirits or gnomes that haunt the ancient megalithic monuments, dolmens, and stone circles of Brittany, a Celtic region of France. They are known as protectors of these ancient sites.

Characteristics and Legends
- Appearance: They are generally described as fearsome and black, with feline claws and feet ending in hooves. Their small red eyes are said to gleam like carbuncles.
- Haunts: Nains primarily reside in and around the ancient stone monuments of Brittany, particularly in the Morbihan region.
- Activities: They are known to dance and hold sacred ceremonies around the dolmens and cairns, especially on Wednesday nights, which is considered their special day. During these ceremonies, they chant the names of the days of the week, excluding Saturday and Sunday, which are sacred to other fairies.
- Interactions with Humans: Nains are generally malevolent toward humans. If a person interrupts their rituals or enters their domain, ill fortune or even death is believed to follow. They are also believed to be forgers of false gold coins that turn into dead leaves when humans try to use them.
- Other Associations: In some traditions, they are associated with a cabalistic alphabet engraved on monuments, which, if deciphered, is said to reveal the location of hidden treasures. Other types of Breton dwarfs/gnomes include the korrigans, crions, courils, and gorics.
The name "Nain" can also refer to a specific dwarf character, Náin son of Grór, from J.R.R. Tolkien's Middle-earth legendarium (e.g., The Hobbit and The Lord of the Rings), who was the lord of the Dwarves of the Iron Hills. However, the folklore meaning is tied to the hills of the Brittany region.