We gathered 37 new spectra of the B star 53 Persei spread over approximately 2 years. On these data, we performed a frequency and a mode analysis. We determined the physical parameters of 53 Persei and placed the star in the theoretical SPB instability strip. We also analysed the 110 photometric observations of the Hipparcos satellite. Our results show the presence of the two frequencies often quoted in the literature nu(1) = 0.461 c/d, nu(2) = 0.594 c/d and a third frequency nu(3) = 0.471 c/d which was first proposed by Mills & Dukes (1994). It is the first time that two studies result in the same third frequency based on two different data sets. The mode analysis provides evidence for l(1) = 1 or l(1) = 2 for the dominant mode, supporting the conclusions of Smith & Huang (1994).