This article is about the landform. For the Tamil saints, see Alvars.

An alvar is a limestone plain with thin soil and, as a result, sparse vegetation. This environment suffers from poor drainage, so these areas flood in the spring and become very dry and hot in the summer. This challenging habitat supports a community of rare plants and animals, including species more commonly found on prairie grasslands. Alvars can be found in southern Sweden, northwest Estonia and around the Great Lakes.

In North America, alvars provide habitat for birds such as Bobolinks, Eastern Meadowlarks, Upland Sandpipers, Eastern Towhees, Brown Thrashers and Loggerhead Shrikes whose habitat is declining elsewhere. Rare plants include Juniper sedge (Carex juniperorum), Lakeside daisy (Hymenoxys acaulis), Ram's-head lady's slipper (Cypripedium arietinum) and Dwarf lake iris (Iris lacustris). Also associated with alvars are rare butterflies and snails.

The use of word "alvar" to refer to this type of environment originated in Scandinavia. A large alvar is located on the Swedish island of Öland. The landscape there has been designated a UNESCO World heritage site.