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We exploit the first \gaia data release to study the properties of theGalactic stellar halo as traced by RR Lyrae. We demonstrate that it is possibleto select a pure sample of RR Lyrae using only photometric informationavailable in the Gaia+2MASS catalogue. The final sample contains about 21600 RRLyrae covering an unprecedented fraction ($\sim60\%$) of the volume of theGalactic inner halo ($\text{R}<28$ kpc). We study the morphology of the stellarhalo by analysing the RR Lyrae distribution with parametric and non-parametrictechniques. Taking advantage of the uniform all-sky coverage, we test halomodels more sophisticated than usually considered in the literature, such asthose with varying flattening, tilt and/or offset of the halo with respect tothe Galactic disc. A consistent picture emerges: the inner halo is wellreproduced by a smooth distribution of stars settled on triaxial ellipsoids.The minor axis is perpendicular to the Milky Ways disc, while the major axis ismisaligned by $\sim20^{\circ}$ from the Galactic Y axis. The elongation alongthe major axis is mild ($\text{p}=1.27$), and the vertical flattening is shownto evolve from a squashed state with $\text{q}\approx0.57$ in the centre to amore spherical $\text{q}\approx0.75$ at the outer edge of our dataset. Thedensity slope is well approximated by a single power-law with exponent$\alpha=-2.96$. Within the range probed, we see no significant evidence for achange of the radial density slope, out of the plane tilt or an offset of thehalo with respect to the Galaxy's centre.

The first all-sky view of the Milky Way stellar halo with Gaia+2MASS RR Lyrae