In particle physics, SO(10) refers to a Grand Unified Theory (GUT) based on the spin group Spin(10). The shortened name SO(10) is conventional among physicists, and derives from the Lie algebra or less precisely the Lie group of SO(10), which is a special orthogonal group that is double covered by Spin(10).

SO(10) subsumes the 1974 Georgi–Glashow and Pati–Salam models, and unifies all fermions in a generation into a single field. This requires 12 new gauge bosons, in addition to the 12 of SU(5) (Georgi–Glashow model) and 9 of SU(4)×SU(2)×SU(2) (Pati–Salam model).

Before the SU(5) theory behind the Georgi–Glashow model, Harald Fritzsch and Peter Minkowski, and independently Howard Georgi, found that all the matter contents are incorporated into a single representation, spinorial 16 of SO(10). However, Georgi found the SO(10) theory just a few hours before finding SU(5) at the end of 1973.

It has the branching rules to [SU(5)×U(1)_χ]/Z₅.

If the hypercharge is contained within SU(5), this is the conventional Georgi–Glashow model, with the 16 as the matter fields, the 10 as the electroweak Higgs field and the 24 within the 45 as the Grand Unified Theory (GUT) Higgs field. The superpotential may then include renormalizable terms of the form Tr(45 ⋅ 45); Tr(45 ⋅ 45 ⋅ 45); 10 ⋅ 45 ⋅ 10, 10 ⋅ 16* ⋅ 16 and 16* ⋅ 16. The first three are responsible to the gauge symmetry breaking at low energies and give the Higgs mass, and the latter two give the matter particles masses and their Yukawa couplings to the Higgs.

There is another possible branching, under which the hypercharge is a linear combination of an SU(5) generator and χ. This is known as flipped SU(5).

Another important subgroup is either [SU(4) × SU(2)_L × SU(2)_R]/Z₂ or Z₂ ⋊ [SU(4) × SU(2)_L × SU(2)_R]/Z₂ depending upon whether or not the left-right symmetry is broken, yielding the Pati–Salam model, whose branching rule is

The symmetry breaking of SO(10) is usually done with a combination of (( a 45_H or a 54_H) and ((a 16_H and a ${\displaystyle {\overline {16}}_{H}}$) or (a 126_H and a ${\displaystyle {\overline {126}}_{H}}$)) ).