Results. We find that WR62a is a double-lined spectroscopic binary with a WN5 primary star and an O5.5-6 type secondary component in orbit with a period of 9.1447 d. The minimum masses range between 21 and 23 M-circle dot for the WN star and between 39 and 42 M-circle dot for the O-type star, thus indicating that the WN star is less massive than the O-type component. We detect a phase shift in the radial-velocity curve of the He II lambda 4686 emission line relative to the other emission line curves. The equivalent width of this emission line shows a minimum value when the WN star passes in front of the system. The analysis of the ASAS photometry confirms the spectroscopic periodicity, presenting a minimum at the same phase.