Although molecular rectifiers were proposed over four decades ago^1,2, until recently reported rectification ratios (RR) were rather moderate^2–11 (RR ~ 10¹). This ceiling was convincingly broken using a eutectic GaIn top contact¹² to probe molecular monolayers of coupled ferrocene groups (RR ~ 10⁵), as well as using scanning tunnelling microscopy-break junctions^13–16 and mechanically controlled break junctions¹⁷ to probe single molecules (RR ~ 10²–10³). Here, we demonstrate a device based on a molecular monolayer in which the RR can be switched by more than three orders of magnitude (between RR ~ 10⁰ and RR ≥ 10³) in response to humidity. As the relative humidity is toggled between 5% and 60%, the current–voltage (I–V) characteristics of a monolayer of di-nuclear Ru-complex molecules reversibly change from symmetric to strongly asymmetric (diode-like). Key to this behaviour is the presence of two localized molecular orbitals in series, which are nearly degenerate in dry circumstances but become misaligned under high humidity conditions, due to the displacement of counter ions (PF₆ ^–). This asymmetric gating of the two relevant localized molecular orbital levels results in humidity-controlled diode-like behaviour.