Subsequent irradiation at 365 nm leads to a 2+2 cycloaddition reaction between the psoralen and opposing thymidine residues, crosslinking the two duplex strands and the oligonucleotide to the DNA. Our strategy harnesses both the functionalisation and crosslinking properties of TFOs to site-specifically modify DNA origami. Moreover, through appropriate design, we show that this approach can be used to reduce the sensitivity of the functionalised origami to thermal denaturation and to disassembly by T7 RNA polymerase. Our strategy is cost-effective as it works with existing DNA origami structures, does not require scaffold redesign, and can be achieved with a single psoralen-modified oligonucleotide. The crosslinking reaction is fast (seconds) and is therefore scalable, energy-efficient, and non-damaging to the structural integrity of the origami scaffold, as well as to fluorescent functionalities introduced into the structure.