Human APOBEC3A is a single-stranded DNA cytidine deaminase that restricts viral pathogens and endogenous retrotransposons, and has a role in the innate immune response. Furthermore, its potential to act as a genomic DNA mutator has implications for a role in carcinogenesis. A deeper understanding of APOBEC3A's deaminase and nucleic acid-binding properties, which is central to its biological activities, has been limited by the lack of structural information. Here we report the nuclear magnetic resonance solution structure of APOBEC3A and show that the critical interface for interaction with single-stranded DNA substrates includes residues extending beyond the catalytic centre. Importantly, by monitoring deaminase activity in real time, we find that A3A displays similar catalytic activity on APOBEC3A-specific TTCA- or A3G-specific CCCA-containing substrates, involving key determinants immediately 5' of the reactive C. Our results afford novel mechanistic insights into APOBEC3A-mediated deamination and provide the structural basis for further molecular studies.