The optical anisotropy in the layer plane of (100) InAs/AlSb superlattices was investigated by ellipsometry and by reflection difference spectroscopy. The superlattices are anisotropic in the (100) plane with the dielectric function along the [011] and [011\ifmmode\bar\else\textasciimacron\fi{}] axes differing by as much as \ensuremath{\sim}5% in the energy range from 2 to 4 eV. The anisotropy in superlattices with either only AlAs or only InSb interface bonds is attributed to a reduction of the ${\mathit{D}}_{2\mathit{d}}$ symmetry of a perfect structure due to differences between the interfaces where InAs is grown on AlSb and those where AlSb is deposited on InAs. Different mechanisms for the interface-related anisotropy are discussed.