A mild visible‐light‐mediated strategy for cysteine arylation is presented. The method relies on the use of eosin Y as a metal‐free photocatalyst and aryldiazonium salts as arylating agents. The reaction can be significantly accelerated in a microflow reactor, whilst allowing the in situ formation of the required diazonium salts. The batch and flow protocol described herein can be applied to obtain a broad series of arylated cysteine derivatives and arylated cysteine‐containing dipeptides. Moreover, the method was applied to the chemoselective arylation of a model peptide in biocompatible reaction conditions (room temperature, phosphate‐buffered saline (PBS) buffer) within a short reaction time. Chemical modifications of peptides: The mild visible‐light‐mediated arylation of cysteine relies on the use of eosin Y as a metal‐free photocatalyst and aryldiazonium salts as arylating agents. The batch and flow method afforded a series of arylated cysteine derivatives and arylated cysteine‐containing dipeptides. A model peptide has been chemoselectively arylated under biocompatible reaction conditions.