  A sharp $L^p$ spectral multiplier theorem of Mihlin--H\"ormander type is
proved for a distinguished sub-Laplacian on quaternionic spheres. This is the
first such result on compact sub-Riemannian manifolds where the horizontal
space has corank greater than one. The proof hinges on the analysis of the
quaternionic spherical harmonic decomposition, of which we present an
elementary derivation.
