A closed wing is a wing that effectively has two main planes that merge at their ends so that there are no conventional wing tips. Closed wing designs include the annular wing (commonly known as the cylindrical or ring wing), the joined wing, the box wing, and spiroid tip devices.

Like many wingtip devices, the closed wing aims to reduce the wasteful effects associated with wingtip vortices that occur at the tips of conventional wings. Although the closed wing has no unique claim on such benefits, many closed wing designs do offer structural advantages over a conventional cantilever monoplane.

Wingtip vortices form a major component of wake turbulence and are associated with induced drag, which is a significant contributor to total drag in most regimes. A closed wing avoids the need for wingtips and thus might be expected to reduce wingtip drag effects.

In addition to potential structural advantages over open cantilevered wings, closed wing surfaces have some unique aerodynamic properties:

The upshot is that although closed systems can produce large induced-drag reductions relative to a conventional planar wing, there is no significant aerodynamic advantage that uniquely accrues to their being closed rather than open.

Various types of closed wing have been described:

An early example of the closed wing was on the Blériot III aircraft, built in 1906 by Louis Blériot and Gabriel Voisin. The lifting surfaces comprised two annular wings mounted in tandem. The later Blériot IV replaced the forward annular wing with a biplane and added a canard foreplane to make it a three-surface aircraft. It was able to leave the ground in small hops before being damaged beyond repair.

Based on the work of G.J.A. Kitchen, Cedric Lee and G. Tilghman Richards built and flew several annular-wing aeroplanes in which the fore and aft segments were on the same level. The first was a biplane. It was followed by a series of monoplanes, the last of the line remaining in use until 1914.