PARP1 is a nuclear protein involved in the maintenance of genomic stability. It catalyses the formation of poly(ADP-ribose) (PAR) to recruit repair proteins at the site of DNA lesions, such as double-strand and single-strand breaks. In the process of DNA replication or repair, there could occur stretch of ssDNA, usually protected by ssDNA binding proteins, but when present in abundance can turn into DNA beaks and cause cell death. PARP1 is an extremely sensitive sensor of DNA breaks; however, the interaction of PARP1 with single-stranded DNA (ssDNA) remains unexplored. Here, we report that the two Zn-fingers, ZnF1 and ZnF2, of PARP1, mediate high-affinity recognition of ssDNA. Our studies suggest that although PAR and ssDNA are chemical analogues, they are recognized by a distinct set of domains of PARP1, yet PAR not only induces dislodging of ssDNA from PARP1 but also hampers the ssDNA-dependent PARP1 activity. It is noteworthy that PAR carrier apoptotic fragment PARP1ΔZnF1–2 gets cleaved from PARP1 to facilitate apoptosis, leaving behind the DNA-bound ZnF1-ZnF2PARP1. Our studies demonstrate that the PARP1ΔZnF1–2 is competent for ssDNA-dependent stimulation only in the presence of another apoptotic fragment ZnF1-ZnF2PARP1, suggesting the indispensability of DNA-bound ZnF1-ZnF2PARP1 dual domains for the same. © 2023 Federation of European Biochemical Societies.