The B2Vn star HR 7355 is found to be a He-rich magnetic star. Spectropolarimetric data were obtained with FORS1 at UT2 on Paranal Observatory to measure the disc-averaged longitudinal magnetic field at various phases of the presumed 0.52 d cycle. A variable magnetic field with strengths between 〈Bz〉 = -2200 and +3200 G was found, with confidence limits of 100 to 130 G. The field topology is that of an oblique dipole, while the star itself is seen about equator-on. In the intensity spectra, the HeI lines show the typical equivalent width (EW) variability of He-strong stars, usually attributed to surface abundance spots. The amplitudes of the EW variability of the HeI lines are extraordinarily strong compared to other cases. These results not only put HR 7355 unambiguously among the early-type magnetic stars, but confirm its outstanding nature: with υ sin i = 320 km s-1, the parameter space in which He-strong stars are known to exist has doubled in terms of rotational velocity. © 2010 The Authors. Journal compilation © 2010 RAS.