  We construct explicit string theory models realizing the recently proposed
"St\"uckelberg Portal" scenario, a framework for building Z' mediation models
without the need to introduce unwanted exotic matter charged under the Standard
Model. This scenario can be viewed purely field-theoretically, although it is
particularly well motivated from string theory. By analyzing carefully the
St\"uckelberg couplings between the Abelian gauge bosons and the RR axions, we
construct the first global intersecting brane models which extend the Standard
Model with a genuine hidden sector, to which it is nonetheless connected via
U(1) mass mixings. Utilizing the explicit models we construct, we discuss some
broad phenomenological properties and experimental implications of this
scenario such as Z-Z' mixings, dark matter stability and relic density, and
supersymmetry mediation. With an appropriate confining hidden sector, our setup
also provides a minimal realization of the hidden valley scenario. We further
explore the possibility of obtaining small Z' masses from a large ensemble of
U(1) bosons. Related to the St\"uckelberg portal are two mechanisms that
connect the visible and the hidden sectors, namely mediation by
non-perturbative operators and the hidden photon scenario, on which we briefly
comment.
