We demonstrate room temperature (RT) polariton lasing in an all-dielectric microcavity containing non-polar III-nitride quantum wells (QWs) as active media. The microcavity is fabricated using the photo-electrochemical etching method, by which an optimally grown m-plane III-nitride active region is detached from the substrate in the form of a membrane, which is subsequently inserted between two SiO2/Ta2O5 distributed Bragg reflectors, with 4 and 10 pairs for the top and bottom mirrors, respectively. The active region consists of 25 GaN/Al0.1Ga0.9N (5 nm/3 nm) QWs. The produced microcavities exhibit two closely spaced polarization-resolved lower polariton branches at RT, in line with the selection rules of the non-polar orientation, having a Rabi splitting of 62 and 72 meV in the E∥a and E∥c polarizations, respectively. In a positively detuned 3λ/2-thick microcavity, polariton lasing is observed at ambient conditions in the E∥a polarization, with a threshold ∼ 3 times lower than previous state of the art, despite the use of a relatively weak top reflector.