Context : Aims : We search for periodic variations in the radial velocity of the young Herbig star HD 135344B with the aim to determine a rotation period . Methods : We analyzed 44 high-resolution optical spectra taken over a time range of 151 days . The spectra were acquired with FEROS at the 2.2m MPG/ESO telescope in La Silla . The stellar parameters of HD 135344B are determined by fitting synthetic spectra to the stellar spectrum . In order to obtain radial velocity measurements , the stellar spectra have been cross-correlated with a theoretical template computed from determined stellar parameters . Results : We report the first direct measurement of the rotation period of a Herbig star from radial-velocity measurements . The rotation period is found to be 0.16 d ( 3.9 hr ) , which makes HD 135344B a rapid rotator at or close to its break-up velocity . The rapid rotation could explain some of the properties of the circumstellar environment of HD 135344B such as the presence of an inner disk with properties ( composition , inclination ) , that are significantly different from the outer disk . Conclusions :