Optical nanoantennas that allow for nanoscale confinement of the electromagnetic fields play a pivotal role in numerous important research fields and application domains relying on optical spectroscopy and photochemical reactions in tightly confined volumes. For example, such nanoantennas have enabled the study of fundamental optical phenomena at the single emitter level (such as organic fluorophore molecules, quantum dots, or nitrogen-vacancy centers in diamonds), in single biomolecule interaction analysis by using labeling with such fluorescent species, 10 or on hot electron-facilitated catalysis. Metallic nanostructures and nanoparticles (NPs) represent key building blocks for the construction of optical nanoantennas. The resonant excitation of localized plasmons (LSPs) on such metallic NPs allows for deep subwavelength confinement of light energy due to collective oscillations of electron density and the associated electromagnetic field. The optical excitation of LSPs is accompanied by a strong enhancement of the incident light intensity, which is not homogeneously distributed around metallic NPs, but occurs at specific spatially distinct regions (e.g. edges/tips or gaps between closely arranged metallic objects) that are commonly referred to as "plasmonic hotspots." 15 Therefore, various strategies to spatially control the attachment of functional chemical moieties at these regions have been pursued for precise control and to maximize the efficiency of their interaction with LSPs. These include the use of photoresist mask windows made by electron beam lithography to overlay with plasmonic hotspots, development of orthogonal surface chemistries, and employment of a static electric field gradient for attracting the species at sharp parts of metallic nanostructures. The use of chemical reactions that are locally triggered by LSPs has been explored based on the LSPgenerated hot electrons and multiphoton absorption-activated linkers for docking of proteins. In addition, LSP-induced polymerization have been shown and intensively used for the preparation of hybrid polymer/metal materials and probing of local chemical reactions. Multiphoton absorption photochemistry has become particularly important in mask-less lithography developed for the preparation of complex structures with a spatial resolution down to a hundred nanometers. Up to now, multiphoton lithography techniques have been utilized for the fabrication of objects with well-controlled three-dimensional geometry serving in diverse areas including micro-optics, 36-38 micro-robotics, 39,40 tissue engineering, and drug delivery to mention a few examples. The vast majority of materials that are structured with the help of multiphoton absorption lithography rely on a polymerization process. Typically, a femtosecond (fs) pulsed laser beam with a near-infrared (NIR) wavelength is focused and scanned through a liquid monomer to initiate polymerization in a small focal volume, where the threshold for initiation is exceeded. locally solidifying the liquid photoresist. Besides multiphoton lithography relying on a polymerization mechanism, a recently reported alternative is based on multiphoton crosslinking (MPC) of soluble polymers employed as solid thin films. Their polymer backbones contain photoactive moieties that were excited by scanning focused NIR beam to induce MPC, resulting in thermoresponsive biofunctional hydrogels. The MPC process was investigated in dependence of the type of crosslinker and its means of conjugation to the polymer backbone. In the present work, we report on a new generic approach to chemically modify narrow zones of metallic nanostructures where plasmonic hotspots occur. The strategy employs polymer chains with incorporated photo-active crosslinkers that can be triggered by the multiphoton absorption process for their simultaneous attachment and crosslinking at the plasmonic hotspot where the MPC threshold is reached. To the best of our knowledge, in contrast to plasmonicallydriven polymerization, this type of plasmonically mediated photochemistry-plasmon-enhanced multiphoton crosslinking (PE-MPC)-has not yet been reported, despite its potential for preparing well-controlled hybrid metal/hydrogel nanomaterials. This method leverages already established multiphoton lithography tools and offers the capability for selective local post-modification of the resulting structures. The reliability of the approach is demonstrated by using two types of polymers that form locally attached hydrogel nanostructures at plasmonic hotspots after swelling in aqueous media. The successful preparation of such materials is documented for optimized writing conditions by atomic force microscopy (AFM) measurements of the local topography and Young's modulus and by characterizing the nanoscopic domains of the thermoresponsive hydrogel matrix with polarization-resolved localized surface plasmon resonance (LSPR) measurements.