  Optical bound states in the continuum (BIC) are peculiar topological states
that, when realized in a planar photonic crystal lattice, are
symmetry-protected from radiating in the far field despite lying within the
light cone, i.e., in the energy-momentum dispersion region for which radiation
can propagate out of the lattice plane. These BICs possess an invariant
topological charge given by the winding number of the polarization vectors,
similarly to vortices in quantum fluids, such as superfluid helium and atomic
Bose-Einstein condensates. In spite of several reports of optical BICs in
patterned dielectric slabs with evidence of lasing, their potential as
topologically protected states with theoretically infinite lifetime has not
been fully exploited, yet. Here we show Bose-Einstein condensation of
polaritons, hybrid light-matter excitations, occuring in a BIC thanks to its
peculiar non-radiative nature. The combination of the ultra-long BIC lifetime
and the tight confinement of the waveguide geometry allow to achieve an
extremely low threshold density for condensation, which is not reached in the
dispersion minimum but at a saddle point in reciprocal space. By bridging
bosonic condensation and symmetry-protected radiation eigenmodes, we unveil new
ways of imparting topological properties onto macroscopic quantum states with
unexplored dispersion features. Such an observation may open a route towards
energy-efficient polariton condensation in cost-effective integrated devices,
ultimately suited for the development of hybrid light-matter optical circuits
