  The double Cayley Grassmannian is a unique smooth equivariant completion with
Picard number one of the 14-dimensional exceptional complex Lie group $G_2$,
and it parametrizes eight-dimensional isotropic subalgebras of the complexified
bi-octonions. We show the rigidity of the double Cayley Grassmannian under
K\"{a}hler deformations. This means that for any smooth projective family of
complex manifolds over a connected base of which one fiber is biholomorphic to
the double Cayley Grassmannian, all other fibers are biholomorphic to the
double Cayley Grassmannian.
