In recreational mathematics, a polyomino is a polyform with the square as its base form. It is a connected shape formed as the union of one or more identical squares in distinct locations on the plane, taken from the regular square tiling, such that every square can be connected to every other square through a sequence of shared edges (i.e., shapes connected only through shared corners of squares are not permitted). Polyominoes with from 1 to 6 squares are called respectively monominoes, dominoes, trominoes (or triominoes), tetrominoes, pentominoes and hexominoes. Polyominoes have been used in popular puzzles since at least 1907, and the enumeration of pentominoes is dated to antiquity. Many results with the pieces of 1 to 6 squares were first published in Fairy Chess Review between the years 1937 to 1957, under the name of "dissection problems". The name polyomino was invented by Solomon W. Golomb in 1953 and they were popularized by Martin Gardner.

In some contexts, the definition of a polyomino is relaxed or refined. Sometimes it is required that polyominoes are simply connected, while on other occasions they may have holes (in other words, regions which are not tiled with squares but which are unconnected to the exterior of the polyomino).

Related to polyominoes are polyiamonds (formed from equilateral triangles), polyhexes (formed from regular hexagons), and other polyforms. Sometimes polyominoes are generalised to three or more dimensions by aggregating cubes (polycube) or hypercubes.

As well as many puzzles in recreational mathematics, polyominoes are a source of combinatorial problems, the most basic being to enumerate polyominoes of a given size. No formula has been found except for special classes of polyominoes. However, a number of estimates are known, and there are algorithms for counting them.