Spatial transcriptomics experiments greatly benefit from brighter signals that improve detection efficiency and shorten imaging times. Here, we introduce a new enzyme-free signal amplification method inspired by the kinetic proofreading principle, in which short oligonucleotide probes are iteratively deposited at their target site and covalently photo-crosslinked to the cell, forming highly stable DNA assemblies. The method results in more than 500-fold signal amplification and supports multiplex readout capabilities.