In geometry, the rhombic dodecahedron is a convex polyhedron with 12 congruent rhombic faces. It has 24 edges, and 14 vertices of 2 types. As a Catalan solid, it is the dual polyhedron of the cuboctahedron. As a parallelohedron, the rhombic dodecahedron can be used to tesselate its copies in space creating a rhombic dodecahedral honeycomb. There are some variations of the rhombic dodecahedron, one of which is the Bilinski dodecahedron. There are some stellations of the rhombic dodecahedron, one of which is the Escher's solid. The rhombic dodecahedron may also appear in nature (such as in the garnet crystal), the architectural philosophies, practical usages, and toys.

The rhombic dodecahedron is a polyhedron with twelve rhombi, each of which long face-diagonal length is exactly ${\displaystyle {\sqrt {2}}}$ times the short face-diagonal length and the acute angle measurement is ${\textstyle \arccos(1/3)\approx 70.53^{\circ }}$. Its dihedral angle between two rhombi is 120°.

The rhombic dodecahedron is a Catalan solid, meaning the dual polyhedron of an Archimedean solid, the cuboctahedron; they share the same symmetry, the octahedral symmetry. It is face-transitive, meaning the symmetry group of the solid acts transitively on its set of faces. In elementary terms, this means that for any two faces, there is a rotation or reflection of the solid that leaves it occupying the same region of space while moving a face to another one. Other than rhombic triacontahedron, it is one of two Catalan solids that each have the property that their isometry groups are edge-transitive; the other convex polyhedron classes being the five Platonic solids and the other two Archimedean solids: its dual polyhedron and icosidodecahedron.

Denoting by a the edge length of a rhombic dodecahedron,

The surface area A and the volume V of the rhombic dodecahedron with edge length a are: $${\displaystyle {\begin{aligned}A&=8{\sqrt {2}}a^{2}&\approx 11.314a^{2},\\V&={\frac {16{\sqrt {3}}}{9}}a^{3}&\approx 3.079a^{3}.\end{aligned}}}$$

The rhombic dodecahedron can be viewed as the convex hull of the union of the vertices of a cube and an octahedron where the edges intersect perpendicularly. The six vertices where four rhombi meet correspond to the vertices of the octahedron, while the eight vertices where three rhombi meet correspond to the vertices of the cube.

The skeleton of a rhombic dodecahedron is called a rhombic dodecahedral graph, with 14 vertices and 24 edges. It is the Levi graph of the Miquel configuration (8₃ 6₄).

For edge length √3, the eight vertices where three faces meet at their obtuse angles have Cartesian coordinates (±1, ±1, ±1). In the case of the coordinates of the six vertices where four faces meet at their acute angles, they are (±2, 0, 0), (0, ±2, 0) and (0, 0, ±2).