  We show on the basis of electronic structure calculations that the uranium
5$f$ magnetic moment in URu$_2$Si$_2$ exhibits a unique Ising behavior, which
surprisingly, arises from itinerant electronic states. The origin of the
unusual Ising behavior is analyzed as due to the peculiar near-Fermi edge
nested electronic structure of URu$_2$Si$_2$ involving its strong spin-orbit
interaction. The Ising anisotropy has pertinent implications for theories
applicable to explaining the Hidden Order phase in URu$_2$Si$_2$.
