We introduce Bi 2 CuO 3 ( SO 4 ) as a rare example of a spin-ladder magnet with ferromagnetic interactions on the rungs. Its magnetic response is studied through measurements of temperature-dependent magnetic susceptibility and heat capacity, field-dependent magnetization, and electron spin resonance spectroscopy. These experiments are complemented by density-functional-theory calculations combined with the construction of maximally localized Wannier functions and an analysis of the relevant superexchange pathways. Quantum Monte Carlo simulations are employed to model thermodynamic properties and to quantitatively determine the magnetic exchange parameters. Our combined approach identifies Bi 2 CuO 3 ( SO 4 ) as a two-leg spin-ladder system with ferromagnetic rungs ( J ′ ≈ − 208 K ) and antiferromagnetic legs ( J ≈ 258 K ). These interactions of similar magnitude arise from remarkably different superexchange pathways, with the Cu–Cu distance along the leg being almost twice as long as the respective distance along the rung. The antiferromagnetic leg coupling represents the strongest oxygen-mediated long-range superexchange in a Cu 2 + compound and sets a benchmark for the role of complex superexchange pathways in quantum magnets.