Many spectral lines in OB stars show unexplained variability on a rotational timescale. This occurs for example in the so-called discrete absorption components (DACs) in UV-wind line profiles and in many optical lines. This variability is considered to be cyclical (like Sun spots), rather than periodic. The absence of strict periodicity is in accordance with the lack of evidence for a permanent magnetic field, with typical upper limits of ≈300 G. We have nevertheless searched for periodicity in 12 years of IUE spectra of the O giant ξ Persei in the N IV 1718 wind line, which is formed close to the star. We find a very stable period of 2.0406 d, which we interpret as due to rotation. Since nonradial pulsations with this period can be ruled out, we predict that ξ Per has a magnetic dipole field. We can predict the most favorable phase to attempt new magnetic measurements.