Extensive photometric UBV observations and spectroscopic high dispersion (20 Å/mm and 38 Å/mm) coudé observations of the very luminous (Mv = −8.2) B2.5 eq supergiant R81 of the LMC were carried out between 1970 and 1980 at ESO, La Silla. In addition the IUE satellite was used to obtain a high resolution (0.2 Å) spectrogram in the ultraviolet wavelength range 1200 < λ < 1950 Å. The most prominent features of the visual spectrum are P Cygni profiles of the Balmer lines, indicating a shell with an expansion velocity of about 140 km s−1. The ultraviolet spectrum of R 81 is dominated by blue-shifted absorption resonance lines (Si II, Si IV, C II, C IV, Al III etc.) and Fe III absorption lines originating from metastable lower levels. From the UV resonance lines a very high mass loss rate (M = 5 10−5 M⊙ yr−1) was estimated. The early Balmer lines show very broad shallow emission wings (total width 40 to 50 Å), attributed to electron scattering. The mass loss is highly variable and presumably occurs in the form of sudden ejections of discrete shells. Irregular brightness variations of a few tenths of a magnitude in V on timescales of weeks were found. An absolute bolometric magnitude Mbol ≈ −10 and a photospheric radius R ≈ 70 R⊙ were estimated. A comparison with theoretical evolutionary tracks indicates a stellar mass M > 50 M⊙. The observed spectroscopic properties lead us to suggest that the LMC star R 81 is a close counterpart of the galactic star P Cyg, representing a short lived transient stage in the evolution of the most massive stars.