A new type of detector for the determination of absolute molecular‐beam intensities is described. The intensity is obtained from the work done on a cylindrical rotating body by the torque exerted by the impinging beam particles. The detector takes advantage of the very low and constant bearing friction of magnetically suspended rotors and of the complete particle momentum accommodation on uncleaned surfaces. The detection limit of the present design isImin0=1.2×1015/Mv˜, whereMis the atom mass andv˜ the average velocity of the beam particles in ms−1. The molecular‐beam detector turns out to be an absolute instrument. Principle, experimental performance, and measurements with helium nozzle beams are reported.