  We obtain distances to a sample of RR Lyrae in the central core of the
Sagittarius dwarf spheroidal galaxy from OGLE data. We use these distances,
along with RR Lyrae from \emph{Gaia} DR2, to measure the shape of the stellar
distribution within the central $\sim$ 2 kpc. The best-fit stellar distribution
is triaxial, with axis ratios 1 : 0.76 : 0.43. A prolate spheroid model is
ruled out at high statistical significance relative to the triaxial model. The
major axis is aligned nearly parallel to the sky plane as seen by an
Earth-based observer and is nearly perpendicular to the direction of the
Galactic center. This result may be compared to cosmological simulations which
generally predict that the major axis of the dark matter distribution of
subhalos is aligned with the Galactic center. The triaxial structure that we
obtain can provide important constraints on the Sagittarius progenitor, as well
as the central dark matter distribution under the assumption of dynamical
equilibrium.
