Small ligands specific to tumor-associated antigens can be used as alternatives to antibodies for the delivery of small payloads such as radionuclides, cytotoxic drugs, and fluorophores. Their use as a delivery moiety of bioactive proteins such as cytokines remains largely unexplored. Here, we describe the preparation and in vivo characterization of the first small molecule-cytokine conjugate targeting carbonic anhydrase IX (CAIX), a marker of renal cell carcinoma and hypoxia. Site-specific conjugation between interleukin-2 and acetazolamide was obtained by sortase A-mediated transpeptidation. Binding of the conjugate to the cognate CAIX antigen was confirmed by surface plasmon resonance. The in vivo targeting of structures expressing carbonic anhydrase IX was assessed by biodistribution experiments in tumor-bearing mice. Optimization of manufacturability and tumor-targeting performance of acetazolamide-cytokine products will be required in order to enable industrial applications.