In a homogeneous two-dimensional system at non-zero temperature there can be no ordering of infinite range 1,2 . However, for a Bose liquid under such conditions, a superfluid phase is predicted 3-5 . Bound vortex-antivortex pairs dominate the thermodynamics and phase coherence properties in this superfluid regime. It is believed that several systems share this behaviour when the parameter describing their ordered state has two degrees of freedom 6 . This theory has been tested for some of them 7-12 , but there has been no direct experimental observation of a quasi-condensate that includes a bound vortex-antivortex pair. Here we present an experimental technique that can identify a single vortex-antivortex pair in a two-dimensional exciton-polariton condensate. The pair is generated through the inhomogeneous spot profile of the pumping laser, and is revealed in the time-integrated phase maps acquired using Michelson interferometry. Numerical modelling based on the open-dissipative Gross-Pitaevskii equation suggests that the pair evolution is distinctly different in this non-equilibrium system compared with atomic condensates 13 .Microcavity exciton polaritons 14 behave as a system of strictly two-dimensional bosons when their density is below the exciton saturation density. As a result of their half-light half-matter nature, their effective mass is extremely small, so that quantum many-body effects are important at relatively high temperatures, even up to room temperature 15 . In particular, dynamic polariton condensation is observed [16][17][18] , and its signatures are similar to Bose-Einstein condensation, namely massive occupation of the ground state and phase coherence up to large distances. However, the short lifetime allows only the formation of a quasi-equilibrium steady state, in which polaritons escaping from the cavity are continuously replenished by the external pump through an intermediate reservoir state.Here, we show that single vortex-antivortex pairs can be observed in an exciton-polariton condensate. We have created a pumping spot that generates a minimum of the condensate density at the centre. A zero in density can be thought of as a superposition of a vortex and antivortex that can be separated by an external perturbation 19 . Thus, the centre of the condensate acts as a source of vortex-antivortex pairs. We have found that for a particular condensate size, there is on average one pair at any time. A Michelson interferometer is used to reconstruct the time-integrated phase map of the system. When the sample disorder potential is stronger than the blueshift induced by polaritonpolariton interactions, pinned pairs appear at certain locations. When the sample disorder potential is weak, on the other hand, they are mobile. They appear along a fixed axis, because of a small asymmetry in the pumping spot, and are created with a random polarization, namely the vortex can appear on the right side of the spot and the antivortex on the left, or vice versa. In the time-integrated measurement, two disti...