In crystallography, the rhombohedral (or trigonal) crystal system is one of the 7 lattice point groups. A crystal system is described by three basis vectors. In the rhombohedral system, the crystal is described by vectors of equal length, all three of which are not mutually orthogonal. The rhombohedral system can be thought of as the cubic system stretched along a body diagonal.

In some classification systems, the rhombohedral system is grouped into a larger hexagonal system.

There exists only one rhombohedral Bravais lattice.

The point groups that fall under this crystal system are listed below, followed by their representations in international notation and Schoenflies notation, and example crystals.

name	international	Schoenflies	example
rhombohedral holohedral		D3d	calcite
rhombohedral hemimorphic	3m	C3v	tourmaline
rhombohedral tetartohedral		S6	dolomite
trapezohedral	32	D3	quartz
rhombohedral tetartohedral	3	C3