Expansion of antibody scaffold diversity has the potential to expand the neutralizing capacity of the immune system and to generate enhanced therapeutics and probes. Systematic exploration of scaffold diversity could be facilitated with a modular and chemical scaffold for assembling proteins, such as DNA. However, such efforts require simple, modular, and site-specific methods for coupling antibody fragments or bioactive proteins to nucleic acids. To address this need, we report a modular approach for conjugating synthetic oligonucleotides to proteins with aldehyde tags at either terminus or internal loops. The resulting conjugates are assembled onto DNA-based scaffolds with low nanometer spatial resolution and can bind to live cells. Thus, this modular and site-specific conjugation strategy provides a new tool for exploring the potential of expanded scaffold diversity in immunoglobulin-based probes and therapeutics.