The broad utility of protein bioconjugates has created a need for new and diverse strategies for site-selective protein modification. In particular, chemical reactions that target alternative amino acid side chains or unnatural functional groups are emerging as a valuable complement to more commonly used lysine- and cysteine-based strategies. Considering their widespread use in organic synthesis, reactions catalyzed by transition metals could provide a particularly powerful set of transformations for the continued expansion of the bioconjugation toolkit. Recent efforts to apply transition metal catalysis to protein modification have resulted in new methods for protein cross-linking, tryptophan modification, tyrosine modification, reductive amination of protein amines, and unnatural amino acid labeling. These strategies have substantially expanded the synthetic flexibility of protein modification, and thus the range of applications for which bioconjugates can be used in chemical biology and materials science.