Mutations in the BK polyomavirus (BKPyV) capsid accumulate in kidney transplant (KTx) recipients with persistent virus replication. They are associated with neutralization escape and appear to arise as a result of cytosine deamination by host cell APOBEC3A/B enzymes. To study the mutagenic processes occurring in patients, we amplified the typing region of the VP1 gene, sequenced the amplicons to a depth of 5000–10,000×, and identified rare mutations, which were fitted to COSMIC mutational signatures. Background mutations were identified in amplicons from plasmids carrying the BKPyV genome and compared to mutations observed in 148 samples from 23 KTx recipients in France and in Vietnam. Three mutational signatures were consistently observed in urine, serum, and kidney biopsy samples, two of which, SBS2 and SBS13, corresponded to APOBEC3A/B activity. In addition, a third signature with no known etiology, SBS89, was detected both in patient samples, and in cells infected in vitro with BKPyV. Quantitatively, APOBEC3A/B mutation rates in urine samples were strongly correlated with urine viral load, and also appeared to vary between individuals. These results confirm that APOBEC3A/B is a major, but not the only, source of BKPyV genome mutations in patients.