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We present for the first time an analysis based on uvby light curves, Hβ indices and on new spectroscopic data of the massive detached double-lined O-type eclipsing binary V 3903Sgr. The uvby light curves are analysed with the WINK (initial solutions) and the Wilson-Devinney (WD, final solution) programs. Both codes were used in their extended versions, with stellar atmospheres and taking into account the geometric distortions and photometric effects caused by proximity of the components. The spectroscopic CCD observations were analysed with the harmonic ``Wilsing-Russell'' and the ``Lehman-Filhes'' methods. We conclude that V 3903Sgr is one of the rare O-type detached systems where both components are still on the initial phases of the main sequence, with an age of either 1.6x10⁶yrs or 2.5x10⁶yrs (depending on the evolutionary model adopted) at a distance of ≃1500pc, the same as for the Lagoon Nebula (Messier 8) complex, of which the system is probably a member. We determine the absolute dimensions: M_A=27.27±0.55, R_A=8.088±0.086, M_B=19.01±0.44 and R_B=6.125±0.060 (solar units). There is no evidence of mass transfer and the system is detached. The orbit is circular, and both components show synchronous rotation, despite their early evolutionary stage. The absolute dimensions determined should be representative for normal single stars. Amongst the massive systems (M>17M_☉) with precise absolute dimensions (errors <2%), V3903 Sgr is that with the most massive primary, with the largest mass difference between the components, and it is the youngest one.