Objective Anti–citrullinated protein antibodies (ACPAs) are a hallmark of rheumatoid arthritis (RA). While epitope spreading of the serum ACPA response is believed to contribute to RA pathogenesis, little is understood regarding how this phenomenon occurs. This study was undertaken to analyze the antibody repertoires of individuals with RA to gain insight into the mechanisms leading to epitope spreading of the serum ACPA response in RA. Methods Plasmablasts from the blood of 6 RA patients were stained with citrullinated peptide tetramers to identify ACPA‐producing B cells by flow cytometry. Plasmablasts were single‐cell sorted and sequenced to obtain antibody repertoires. Sixty‐nine antibodies were recombinantly expressed, and their anticitrulline reactivities were characterized using a cyclic citrullinated peptide enzyme‐linked immuosorbent assay and synovial antigen arrays. Thirty‐six mutated antibodies designed either to represent ancestral antibodies or to test paratope residues critical for binding, as determined from molecular modeling studies, were also tested for anticitrulline reactivities. Results Clonally related monoclonal ACPAs and their shared ancestral antibodies each exhibited differential reactivity against citrullinated antigens. Molecular modeling identified residues within the complementarity‐determining region loops and framework regions predicted to be important for citrullinated antigen binding. Affinity maturation resulted in mutations of these key residues, which conferred binding to different citrullinated epitopes and/or increased polyreactivity to citrullinated epitopes. Conclusion These results demonstrate that the different somatic hypermutations accumulated by clonally related B cells during affinity maturation alter the antibody paratope to mediate epitope spreading and polyreactivity of the ACPA response in RA, suggesting that these may be key properties that likely contribute to the pathogenicity of ACPAs.