A prominent outburst of the flat spectrum radio quasar 3C 454.3 was observed in 2014 June with the 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 $F[E\gt 100\,\mathrm{MeV}]=(7.2\pm 0.2)\times {10}^{-6}$ ph cm(−)(2) s(−)(1), with a spectral index, for a simple power law, of ${\rm{\Gamma }}=2.04\pm 0.01$. That made this outburst the first γ-ray high state of 3C 454.3 ever to be detected by 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 $F[E\gt 100\,\mathrm{MeV}]=(17.6\pm 1.9)\times {10}^{-6}$ 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 × 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 γ-ray emission region to be located close to the outer boundary of the broad-line region, leading to fast flux variability.Read less <