In geometry, an equilateral polygon is a polygon which has all sides of the same length.

For instance, an equilateral triangle is a triangle of equal edge lengths. All equilateral triangles are similar to each other, and have 60 degree internal angles.

An equilateral quadrilateral is a rhombus, which includes the square as a special case.

An equilateral polygon which is cyclic (its vertices are on a circle) is a regular polygon (a polygon that is both equilateral and equiangular).

All equilateral quadrilaterals are convex, but there exist equilateral polygons with five sides (pentagons) which are concave, and similarly for every larger number of sides.

Viviani's theorem generalizes to equilateral polygons.^{[citation needed]}

Triambi, which are equilateral hexagons with trigonal symmetry, appear in the three triambic icosahedra:

Small triambic icosahedron

Medial triambic icosahedron

Great triambic icosahedron