A mara or mare is a kind of malignant female wraith in Scandinavian folklore believed to cause nightmares. She appears as early as in the Old Norse sagas, but the belief itself is probably of much older date (see below). "Mara" is the Old Norse and Swedish name, "Mare" is Norwegian and Danish.

The mara was thought of as an immaterial being – capable of moving through a keyhole or the opening under a door – who seated herself at the chest of a sleeping person and "rode" him or her, thus causing nightmares. In Norwegian/Danish, the word for nightmare is mareritt/mareridt or "mareride" (the Swedish word being mardröm, "maredream"). The weight of the mara could also result in breathing difficulties or feeling of suffocation (see: sleep paralysis).

The mara was also believed to "ride" horses, which left them exhausted and covered in sweat by the morning. She could also entangle the hair of the sleeping man or beast, resulting in "marelocks" (see: Polish plait). Even trees could be ridden by the mara, resulting in branches being entangled. The undersized, twisted pine-trees growing on costal rocks and on wet grounds are known in Sweden as martallar (marepines).

According to a common belief, the mara was the free-roaming spirit of sleeping women, who became maras either out of wickedness or as a form of curse. In the latter case, finding out whom the cursed person was and simply say: "You are a mara," was often enough to release her from this condition.

The word ultimately derives from the Indo-European root *mer-, meaning to rub away or to harm. The concept of the mara has very old roots in the folklore of the Germanic peoples. Indeed the English word nightmare stems from the Anglo-Saxon and Old English belief in this creature. In English folklore, Hags and Witches later took on many of the roles of the mara. In Germany the activities of the mara where shifted to the elves (nightmare in German is alptraum or "elf dream").

Similar mythical creatures are the succubus/incubus, although the belief in the mara completely lacks the fundamental sexual element of these beings.