We present the results of a detailed analysis of 121 ground-based high-resolution, high signal-to-noise ratio spectroscopic measurements spread over 3 yr for the β Cephei star θ Ophiuchi. We discovered θ Oph to be a triple system. In addition to the already known speckle B5 companion of the B2 primary, we showed the presence of a low-mass spectroscopic companion and we derived an orbital period of 56.71 d with an eccentricity of 0.1670. After removing the orbit we determined two frequencies for the primary in the residual radial velocities: f1= 7.1160 cd-1 and f2= 7.4676 cd-1. We also found the presence of f3= 7.3696 cd-1 by means of a two-dimensional frequency search across the SiIII 4567-Åprofiles. We identified the m-value of the main mode with frequency f1 by taking into account the photometric identifications of the degrees l. By means of the moment method and the amplitude and phase variations across the line profile, we derived (l1, m1) = (2, -1). This result allows us to fix the mode identifications of the whole quintuplet for which three components were detected in photometry. This is of particular use for our forthcoming seismic modelling of the primary. We also determined stellar parameters of the primary by non-local thermodynamic equilibrium hydrogen, helium and silicon line profile fitting and we obtained Teff= 24000 K and logg= 4.1, which is consistent with photometrically determined values.