We present the results of magnetometric observations of the classic high mass X-ray binary star X Persei. Analysing magnetically-induced circular polarization in the strong emission spectral features, we have detected the presence of a variable longitudinal magnetic field in this star. The field varies from -413 ±100 G to +491± 130 G with the average value -161±113 G and period P ≍ 0.7 days. We interpret this period to be due to the spin rotation of the star. A combined analysis of absorption spectral lines revealed no field.