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Cell surface engineering with functional polymers is an effective strategy to modulate cell activity. Here, a bio‐palladium catalyzed polymerization strategy was developed for in situ synthesis of conjugated polymers on living cell surfaces. Through Sonagashira polymerization, photoactive polyphenyleneethynylene (PPE) is synthesized on the cell surface via cell‐generated bio‐Pd catalyst. The in situ formed PPE is identified by excellent light‐harvest capacity and blue fluorescence on the surfaces of E. coli and C. pyrenoidosa. Besides imaging microbes for tracing the polymerization process, PPE also exhibits enhanced antibacterial activity against E. coli. It can also augment the ATP synthesis of C. pyrenoidosa through enlarging the light absorption and accelerating the cyclic electron transport of the algae. With this bio‐metal catalyzed polymerization method, functional polymers can be synthesized in situ on the living cell surface.