Context: Most current radial velocity planet search programs have concentrated on stars of one solar mass. Our knowledge on the frequency of giant planets and brown dwarf companions to more massive stars is thus rather limited. In the case of solar-like stars, the frequency of short-period brown dwarf companions and very massive planets seems to be low. Aims: Here we present evidence for a substellar companion to 30 Ari B, an F-star of 1.16±0.04 M_&sun; that is a member of a hierarchical triple system. Methods: The companion was detected by means of precise radial velocity measurements using the 2-m Alfred-Jensch telescope and its échelle spectrograph. An iodine absorption cell provided the wavelength reference for precise stellar radial velocity measurements. Results: We analyzed our radial velocity measurements and derived an orbit to the companion with period, P = 335.1±2.5 days, eccentricity e = 0.289±0.092, and mass function f(m) = (6.1±1.7)×10-7 M_&sun;. Conclusions: We conclude that the radial velocity variations of 30 Ari B are due to a companion with m sin i of 9.88±0.94 MJup that is either a massive planet or a brown dwarf. The object thus belongs to the rare class of massive planets and brown dwarfs orbiting main- sequence stars. Based on observations obtained at the 2-m Alfred-Jensch telescope at the Thüringer Landessternwarte Tautenburg. Table 3 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/507/1659 Visitor at the Instituto de Astrofísica de Canarias, C/ Vía Láctea, s/n, 38205 La Laguna (Tenerife), Spain.