We present the results of a spectroscopic multisite campaign for the β Cephei star 12 (DD) Lacertae. Our study is based on more than thousand high-resolution high S/N spectra gathered with eight different telescopes in a time span of 11 months. In addition, we make use of numerous archival spectroscopic measurements. We confirm 10 independent frequencies recently discovered from photometry, as well as harmonics and combination frequencies. In particular, the slowly pulsating B-stars (SPB)-like g-mode with frequency 0.3428d-1 reported before is detected in our spectroscopy. We identify the four main modes as (l1, m1) = (1, 1), (l2, m2) = (0, 0), (l3, m3) = (1, 0) and (l4, m4) = (2, 1) for f1 = 5.178964d-1, f2 = 5.334224d-1, f3 = 5.066316d-1 and f4 = 5.490133d-1, respectively. Our seismic modelling shows that f2 is likely the radial first overtone and that the core overshooting parameter αov is lower than 0.4 local pressure scale heights.