We report the discovery of a hot Jupiter on a 3.28-day orbit around a 1.08M_{sun}_ G0 star that is the secondary component in a loose binary system. Based on follow-up radial velocity observations of TOI-858 B with CORALIE on the Swiss 1.2m telescope and CHIRON on the 1.5m telescope at the Cerro Tololo Inter-American Observatory (CTIO), we measured the planet mass to be 1.10+/-0.08M_Jup_. Two transits were further observed with CORALIE to determine the alignment of TOI-858 B b with respect to its host star. Analysis of the Rossiter-McLaughlin signal from the planet shows that the sky-projected obliquity is lambda=99.3+/-3.8. Numerical simulations show that the neighbour star TOI-858 A is too distant to have trapped the planet in a Kozai-Lidov resonance, suggesting a different dynamical evolution or a primordial origin to explain this misalignment. The 1.15M_{sun}_ primary F9 star of the system (TYC 8501-01597-1, at rho~11'') was also observed with CORALIE in order to provide upper limits for the presence of a planetary companion orbiting that star.