Nonreciprocal Cooper-pair transport is central to superconducting diode devices. Existing realizations rely on asymmetric critical currents induced by magnetic fields, ferromagnets, or complex heterostructures. Here we demonstrate a fundamentally different mechanism for rectifying Cooper pair currents that arises solely from electron–electron interactions in nanoscale superconducting lead islands. Scanning tunneling microscopy reveals Cooper-pair transport through resonant charge states under Coulomb blockade. Similar to a p–n junction, rectification arises in this system from breaking both inversion symmetry, in asymmetric double junctions, and particle–hole symmetry, by gating a charge offset, thereby achieving a gate-switchable Cooper-pair diode without external magnetic field. We tested this effect by rectifying Cooper-pair currents and by broadband microwave photodetection. These results establish Coulomb interactions as a field-free route to nonreciprocal Cooper-pair transport.