Nature Materials, Published online: 28 October 2026; doi:10.1038/s41563-026-02786-1Epitaxial strain and a metallic SrRuO3 bottom electrode are used to stablilize a monoclinic phase in four-unit-cell BiFeO3 thin films. A broad suite of measurements are used to support the presence of both ferroelectricity and d-wave altermagnetism in these ultrathin films.