  A prominent outburst of the flat spectrum radio quasar 3C~454.3 was observed
in 2014 June with the \emph{Fermi} Large Area Telescope. This outburst was
characterized by a three-stage light-curve pattern---plateau, flare and
post-flare---that occurred from 2014 May to July, in a similar pattern as
observed during the exceptional outburst in 2010 November. The highest flux of
the outburst reported in this paper occurred during 2014 June 7--29, showing a
multiple-peak structure in the light-curves. The average flux in these 22 days
was found to be $F[E > 100~\mathrm{MeV}] = (7.2 \pm 0.2) \times
10^{-6}$~ph~cm$^{-2}$~s$^{-1}$, with a spectral index, for a simple power law,
of $\Gamma = 2.04 \pm 0.01$. That made this outburst the first $\gamma$-ray
high state of 3C~454.3 ever to be detected by \emph{Fermi} with such a hard
spectrum over several days. The highest flux was recorded on 2014 June 15, in a
3 hr bin, at MJD 56823.5625, at a level of $F[E > 100~\mathrm{MeV}] = (17.6 \pm
1.9) \times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$. The rise time of one of the short
subflares was found to be $T_r= 1200 \pm 700$~s at MJD = 56827, when the flux
increased from 4 to 12 $\times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$. Several photons
above 20 GeV were collected during this outburst, including one at 45 GeV on
MJD 56827, constraining the $\gamma$-ray emission region to be located close to
the outer boundary of the broad-line region, leading to fast flux variability.
