The site-selective introduction of photo-crosslinking groups into proteins enables the discovery and mapping of weak and/or transient protein interactions with high spatiotemporal resolution, both in vitro and in vivo. We report the genetic encoding of a furan-based, photo-crosslinking amino acid in human cells; it can be activated with red light, thus offering high penetration depths in biological samples. This is achieved by activation of the amino acid and charging to its cognate tRNA by a pyrrolysyl-tRNA-synthetase (PylRS) mutant with broad polyspecificity. To gain insights into the recognition of this amino acid and to provide a rationale for its polyspecificity, we solved three crystal structures of the PylRS mutant: in its apo-form, in complex with adenosine 5′-(β,γ-imido)triphosphate (AMP-PNP) and in complex with the AMP ester of the furan amino acid. These structures provide clues for the observed polyspecificity and represent a promising starting point for the engineering of PylRS mutants with further increased substrate scope.