Two-photon polymerization (2PP) enables the fabrication of complex 3D nano- and microstructures with excellent design freedom and is therefore useful for a variety of research areas like photonics. Yet, process monitoring of the fabrication remains challenging due to the extremely small structure sizes and complex development process. Our study introduces digital holographic microscopy (DHM) as an in-line metrology method for 2PP manufacturing. The DHM procedure involves capturing interference patterns emerging from the overlap of object and reference paths of the laser beam. The object path conveys valuable height information through phase shifts caused by the structure. This technique has excellent advantages, e.g., fast measurement without disruption of the fabrication process. The promising application of such technology is real-time optimization during fabrication with high compliance to predefined specifications. This innovation enhances process control and quality assurance for 3D micro/nanostructures made by 2PP, closing a critical gap in nanofabrication.