A baluster is an upright support, often a vertical moulded shaft, square, or lathe-turned form found in stairways, parapets, and other architectural features. In furniture construction it is known as a spindle. Common materials used in its construction are wood, stone, and less frequently metal and ceramic. A group of balusters supporting a handrail, coping, or ornamental detail are known as a balustrade.

Balusters may be made of carved stone, cast stone, plaster, polymer, polyurethane/polystyrene, polyvinyl chloride (PVC), precast concrete, wood or wrought iron. Cast-stone balusters were a development of the 18th century in Great Britain (see Coade stone), and cast iron balusters a development largely of the 1840s.

The baluster, being a turned structure, tends to follow design precedents that were set in woodworking and ceramic practices, where the turner's lathe and the potter's wheel are ancient tools. The profile a baluster takes is often diagnostic of a particular style of architecture or furniture, and may offer a rough guide to date of a design, though not of a particular example.

Balusters are normally separated by at least the same measurement as the size of the square bottom section. Placing balusters too far apart diminishes their aesthetic appeal, and the structural integrity of the balustrade they form. Balustrades normally terminate in heavy newel posts, columns, and building walls for structural support.