We exploit the first Gaia data release to study the properties of the Galactic stellar halo as traced by RR Lyrae .
We demonstrate that it is possible to select a pure sample of RR Lyrae using only photometric information available in the Gaia +2MASS catalogue .
The final sample contains about 21600 RR Lyrae covering an unprecedented fraction ( \sim 60 \% ) of the volume of the Galactic inner halo ( \text { R } < 28 kpc ) .
We study the morphology of the stellar halo by analysing the RR Lyrae distribution with parametric and non-parametric techniques .
Taking advantage of the uniform all-sky coverage , we test halo models more sophisticated than usually considered in the literature , such as those with varying flattening , tilt and/or offset of the halo with respect to the Galactic disc .
A consistent picture emerges : the inner halo is well reproduced by a smooth distribution of stars settled on triaxial density ellipsoids .
The shortest axis is perpendicular to the Milky Way ’ s disc , while the longest axis forms an angle of \sim 70 ^ { \circ } with the axis connecting the Sun and the Galactic centre .
The elongation along the major axis is mild ( \text { p } = 1.27 ) , and the vertical flattening is shown to evolve from a squashed state with \text { q } \approx 0.57 in the centre to a more spherical \text { q } \approx 0.75 at the outer edge of our dataset .
Within the radial range probed , the density profile of the stellar halo is well approximated by a single power-law with exponent \alpha = -2.96 .
We do not find evidence of tilt or offset of the halo with respect to the Galaxy ’ s disc .