Context : HD 150136 is one of the nearest systems harbouring an O3 star .
Although this system was for a long time considered as binary , more recent investigations have suggested the possible existence of a third component .

Aims : We present a detailed analysis of HD 150136 to confirm the triple nature of this system .
In addition , we investigate the physical properties of the individual components of this system .

Methods : We analysed high-resolution , high signal-to-noise data collected through multi-epoch runs spread over ten years .
We applied a disentangling program to refine the radial velocities and to obtain the individual spectra of each star .
With the radial velocities , we computed the orbital solution of the inner system , and we describe the main properties of the orbit of the outer star such as the preliminary mass ratio , the eccentricity , and the orbital-period range .
With the individual spectra , we determined the stellar parameters of each star by means of the CMFGEN atmosphere code .

Results : We offer clear evidence that HD 150136 is a triple system composed of an O3V ( ( f ^ { * } ) ) –3.5V ( ( f ^ { + } ) ) , an O5.5–6V ( ( f ) ) , and an O6.5–7V ( ( f ) ) star .
The three stars are between 0–3 Myr old .
We derive dynamical masses of about 64 , 40 , and 35 M _ { \odot } for the primary , the secondary and the third components by assuming an inclination of 49 \degr ( \sin ^ { 3 } { i } = 0.43 ) .
It currently corresponds to one of the most massive systems in our galaxy .
The third star moves with a period in the range of 2950 to 5500 d on an outer orbit with an eccentricity of at least 0.3 .
However , because of the long orbital period , our dataset is not sufficient to constrain the orbital solution of the tertiary component with high accuracy .

Conclusions : We confirm the presence of a tertiary star in the spectrum of HD 150136 and show that it is physically bound to the inner binary system .
This discovery makes HD 150136 the first confirmed triple system with an O3 primary star .