SignificancePARP [Poly(ADP-ribose) polymerase] proteins are important signaling enzymes in eukaryotic DNA damage repair. We investigated how PARP1 and PARP2 bind at DNA double-strand breaks (DSBs) by developing a single-molecule magnetic tweezers assay to measure protein binding at DSBs while controlling DNA tension, supercoiling, and end-chemistry. We found that PARP2 forms an extremely stable mechanical bridge across blunt DNA ends and restores DNA torsional continuity. We also discovered that PARP2 switches between end-binding and bridging modes depending on whether overhangs have 5′ or 3′. PARP1, in contrast, binds at DSB ends but does not form a bridge when opposing ends are brought together. Our results reveal fundamental mechanisms of how PARP proteins engage at DSBs, which may aid in inhibitor development.