Mirror-enhanced 4Pi-SMLM with one objective enables isotropic nanoscale imaging.

Dual-objective 4Pi single-molecule localization microscopy (4Pi-SMLM) offers isotropic nanoscale resolution; however, its broader adoption is limited by instrumental complexity and stringent alignment requirements. Here we introduce mirror-enhanced 4Pi-SMLM (me4Pi-SMLM), a single-objective configuration that uses mirror-based retroreflection of the illumination beam to generate phase-tunable interference fringes. This design improves the axial resolution of astigmatism-based methods by approximately fivefold, delivering performance comparable to conventional 4Pi-SMLM while greatly reducing system complexity and maintenance. me4Pi-SMLM achieves near-isotropic localization precision of 2-3 nm in biological samples, enabling clear and unambiguous visualization of diverse ultrastructural features. Furthermore, it achieves sub-15 nm isotropic resolution in brain slices and facilitates high-fidelity two-colour imaging, nanoscale whole-cell reconstruction and live-cell imaging. me4Pi-SMLM can be seamlessly integrated into existing 3D-SMLM systems, enhancing performance with minimal cost and effort.