A Bose-Einstein condensate (BEC) is a state of matter in which extensive collective coherence leads to intriguing macroscopic quantum phenomena(1). In crystalline semiconductor microcavities, bosonic quasiparticles, known as exciton-polaritons, can be created through strong coupling between bound electron-hole pairs and the photon field(2). Recently, a non-equilibrium BEC (ref. 3) and superfluidity(4,5) have been demonstrated in such structures. With organic crystals grown inside dielectric microcavities, signatures of polariton lasing have been observed(6). However, owing to the deleterious effects of disorder and material imperfection on the condensed phase(7-9), only crystalline materials of the highest quality have been used until now. Here we demonstrate non-equilibrium BEC of exciton-polaritons in a polymer-filled microcavity at room temperature. We observe thermalization of polaritons and, above a critical excitation density, clear evidence of condensation at zero in-plane momentum, namely nonlinear behaviour, blueshifted emission and long-range coherence. The key signatures distinguishing the behaviour from conventional photon lasing are presented. As no crystal growth is involved, our approach radically reduces the complexity of experiments to investigate BEC physics and paves the way for a new generation of opto-electronic devices, taking advantage of the processability and flexibility of polymers.