  Superfluidity---the suppression of scattering in a quantum fluid at
velocities below a critical value---is one of the most striking manifestations
of the collective behaviour typical of Bose-Einstein condensates. This
phenomenon, akin to superconductivity in metals, has until now only been
observed at prohibitively low cryogenic temperatures. For atoms, this limit is
imposed by the small thermal de Broglie wavelength, which is inversely related
to the particle mass. Even in the case of ultralight quasiparticles such as
exciton-polaritons, superfluidity has only been demonstrated at liquid helium
temperatures. In this case, the limit is not imposed by the mass, but instead
by the small exciton binding energy of Wannier-Mott excitons, which places the
upper temperature limit. Here we demonstrate a transition from normal to
superfluid flow in an organic microcavity supporting stable Frenkel
exciton-polaritons at room temperature. This result paves the way not only to
table-top studies of quantum hydrodynamics, but also to room-temperature
polariton devices that can be robustly protected from scattering.
