Context : Strong meridional mixing induced by rapid rotation is one reason why all hot main-sequence stars are not chemically peculiar . However , the finding that the He-strong CP star HR 7355 is a rapid rotator complicates this concept . Aims : Our goal is to explain the observed behaviour of HR 7355 based on period analysis of all available photometry . Methods : Over two years , we acquired 114 new BV observations of HR 7355 at observatories in Arizona , U.S.A and Cape Town , South Africa . We performed period analyses of the new observations along with new analyses of 732 archival measurements from the Hipparcos and ASAS projects . Results : We find that the light curves of HR 7355 in various filters are quite similar , with amplitudes 0.035 ( 4 ) , 0.036 ( 4 ) , and 0.038 ( 3 ) mag in B , Hp , and V , respectively . The light curves are double-peaked , with unevenly deep minima . We substantially refine the rotational period to be P = 0 \aas@@fstack { d } 5214410 ( 4 ) , indicating that HR 7355 is the most rapidly rotating CP star known . Our period analyses reveal a possible lengthening of the rotational period with \dot { P } / P = 2.4 ( 8 ) \times 10 ^ { -6 } yr ^ { -1 } . Conclusions : We conclude that the shape and amplitude of HR 7355 light curves are typical of magnetic He-strong CP stars , for which light variations are the result of photospheric spots on the surface of a rotating star . We hypothesise that the light variations are caused mainly by an uneven distribution of overabundant helium on the star ’ s surface . We briefly describe and discuss the cause of the rapid rotational braking of the star .