We study the impact of a magnetic field, generated in collisions of relativistic heavy ions, on the decay probability of a quarkonium produced in the central rapidity region. The quark and antiquark components are subject to mutually orthogonal electric and magnetic fields in the quarkonium comoving frame. In the presence of an electric field, the quarkonium has a finite dissociation probability.We use theWKB approximation to derive the dissociation probability. We find that the quarkonium dissociation energy, i.e., the binding energy at which the dissociation probability is of order unity, increases with the magnetic field strength. It also increases with quarkonium momentum in the laboratory frame owing to the Lorentz boost of the electric field in the comoving frame. We argue that J/ψ’s produced in heavy-ion collisions at the Large Hadron Collider with P⊥ > 9 GeV would dissociate even in vacuum. In plasma, the J/ψ dissociation in a magnetic field is much stronger because of the decrease of its binding energy with temperature.We discuss phenomenological implications of our results.