  We show the presence of a potentially detectable gamma-ray line in the Fermi
Galactic center excess in light of the secluded (vector) dark matter (DM) model
in which the hidden scalar, nearly degenerate with DM in mass, mediates the
interaction of the secluded DM with the Standard Model (SM) due to its mixing
with the SM Higgs. We find that the parameter region $m_X\in[60, 132]$ GeV can
provide a good fit to the Fermi Galactic center gamma-ray excess spectrum,
appearing a prominent gamma-ray line with the energy $\in [30, 66]$ GeV. The
best fit gives $m_X\simeq m_S \simeq 86$ GeV with a $p$-value$\, =0.42$, so
that the resultant gamma-ray line, arising from the decay of the scalar
mediator into $\gamma\gamma$, peaks at 43 GeV. We derive constraints on the
annihilation cross section from the Fermi-LAT gamma-ray line search, gamma-ray
observations of the Fermi-LAT dwarf spheroidal galaxies, and Planck cosmic
microwave background measurement. For the secluded vector DM model, the
parameter space constrained by the current XENON1T and future LUX-ZEPLIN is
shown. Finally, for the mixing angle between the Higgs sectors, we discuss its
lower bound, which is required by the big bang nucleosynthesis constraint and
relevant to the hidden sector decoupling temperature.
