In this work we report the discovery of a new Galactic O2 If*/WN6 star, a rare member of the extremely massive hydrogen core-burning group of stars that, because of their high intrinsic luminosity (close to the Eddington limit), possess an emission-line spectrum at the beginning of their main-sequence evolution, mimicking the spectral appearance of classical Wolf–Rayet (WR) stars. The new star is named WR 42e and is found in isolation at 2.7 arcmin (∼6 pc) from the core of the starburst cluster NGC 3603. From the computed E(B−V) colour excess and observed visual magnitude it is possible to estimate its absolute visual magnitude as MV=−6.3 mag, which is a value similar to those obtained by other researchers for stars of similar spectral type both in the Galaxy and in the Large Magellanic Cloud. Considering the derived absolute visual magnitude, we compute a bolometric stellar luminosity of about 3.2 × 106 L⊙. Finally, we estimate the mass of the new O2 If*/WN6 star by comparing its observed magnitudes and colours with those of other probable NGC 3603 cluster members, finding that the initial mass of WR 42e possibly exceeds 100 M⊙.