The functionalisation of peptides at a late synthesis stage holds great potential, for example, for the synthesis of peptide pharmaceuticals, fluorescent biosensors or peptidomimetics. Here we describe an on‐resin iodination‐substitution reaction sequence on homoserine that is also suitable for peptide modification in a combinatorial format. The reaction sequence is accessible to a wide range of sulfur nucleophiles with various functional groups including boronic acids, hydroxy groups or aromatic amines. In this way, methionine‐like thioethers or thioesters and thiosulfonates are accessible. Next to sulfur nucleophiles, selenols, pyridines and carboxylic acids were successfully used as nucleophiles, whereas phenols did not react. The late‐stage iodination‐substitution approach is not only applicable to short peptides but also to the more complex 34‐amino‐acid WW domains. We applied this strategy to introduce 7‐mercapto‐4‐methylcoumarin into a switchable ZnII responsive WW domain to design an iFRET‐based ZnII sensor.