We report the discovery of a hot Jupiter on a 3.28-day orbit around a 1.08 M_⊙ 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 1.2 m telescope and CHIRON on the 1.5 m telescope at the Cerro Tololo Inter-American Observatory (CTIO), we measured the planet mass to be 1.10_−0.07^+0.08 M_J. 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 λ = 99.3_−3.7^+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 1.15 M_⊙ primary F9 star of the system (TYC 8501-01597-1, at ρ ~11″) was also observed with CORALIE in order to provide upper limits for the presence of aplanetary companion orbiting that star.