The Hipparcos Space Astrometry Mission photometric observations of V398 Lac, led to the discovery of its variability, allowing to classify it as an eclipsing binary with an orbital period of about 5.4 days. This prompted us to acquire high-resolution echelle spectra with the aim of performing accurate radial velocity measurements and to determine the main physical parameters of the system's components. We present, for the first time, a double-lined radial velocity curve and determine the orbital and physical parameters of the two components, that can be classified both as late B-type stars. In particular, we obtained an orbital inclination i similar to 85 degrees. With this value of the inclination, we deduced masses M-1 = 3.83 +/- 0.35 M-circle dot and M-2 = 3.29 +/- 0.32 M-circle dot, and radii R-1 = 4.89 +/- 0.18 R-circle dot and R-2 = 2.45 +/- 0.11 R-circle dot for the more massive and less massive components, respectively. Both components are well inside their own Roche lobes. The mass ratio is M-2/M-1, similar to 0.86. We derived also the projected rotational velocities as v(1) sin i = 79 +/- 2 km s(-1) and v(2) Sin i = 19 +/- 2 km s(-1). Our measurements indicate that the rotation of the primary star is essentially pseudo-synchronized with the orbital velocity at the periastron, while the secondary appears to rotate very slowly and has not yet attained synchronization. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.