This study elucidates the mechanism of single-stranded DNA (ssDNA) annealing by the RAD52-RPA complex. RAD52 is essential for DNA repair, mitotic DNA synthesis, and telomere maintenance. It promotes ssDNA annealing and functions as an alternative to BRCA2/RAD51-dependent homologous recombination. Inactivation of RAD52 in BRCA1/2-deficient cells is synthetically lethal, and aberrant RAD52 expression is linked to poor cancer prognosis. RAD52 is thus a promising therapeutic target for homologous-recombination-deficient cancers. The study reveals that RAD52 forms undecameric (11-subunit) ring structures, but these are not the active form. Instead, open rings, in association with RPA, drive ssDNA annealing. Cryo-electron microscopy and biochemical analyses show that ssDNA binds to the positively charged channel of the open ring. The N-terminal domains of RAD52 drive annealing, while the C-terminal regions modulate the open-ring conformation and RPA interaction. RPA binds to RAD52 at the site of ring opening, with critical interactions between the RPA-interacting domain of RAD52 and the winged helix domain of RPA2. The structures of RAD52 closed and open rings were determined using cryo-EM. The open rings contain up to ten subunits, while the closed rings have eleven. The ssDNA-binding domain (DBD) of RAD52 is flexible and stabilizes ssDNA in the positively charged groove of the ring. The RAD52-ssDNA complex shows that ssDNA is stretched non-uniformly, with each quartet adopting the length of B-form DNA. Arg153 coordinates the phosphate backbone of the two central nucleotides, while Arg55 and Lys152 coordinate between quartet stacks. Mg²+ and surrounding water molecules are coordinated by residues from neighboring subunits. The oligomeric state of human RAD52 is influenced by protein concentration but not by ssDNA binding. The C-terminal region of RAD52 is structurally disordered but plays a critical role in RPA interaction and nuclear localization. Deletion mutants of RAD52 show that the RPA-interacting domain (RID) and C-terminal regions are essential for ring conformation and RPA interaction. The RID and C-terminal sequences are highly conserved in vertebrates. RAD52 open rings form more stable complexes with ssDNA and RPA compared to closed rings, explaining why RAD52-CRs do not cooperate with RPA in ssDNA annealing. The cryo-EM structure of the RPA-ssDNA complex reveals that RPA binds to the positively charged groove of RPA1 and RPA2. The RAD52-ssDNA-RPA complex shows that RPA binds at the site of ring opening, andThis study elucidates the mechanism of single-stranded DNA (ssDNA) annealing by the RAD52-RPA complex. RAD52 is essential for DNA repair, mitotic DNA synthesis, and telomere maintenance. It promotes ssDNA annealing and functions as an alternative to BRCA2/RAD51-dependent homologous recombination. Inactivation of RAD52 in BRCA1/2-deficient cells is synthetically lethal, and aberrant RAD52 expression is linked to poor cancer prognosis. RAD52 is thus a promising therapeutic target for homologous-recombination-deficient cancers. The study reveals that RAD52 forms undecameric (11-subunit) ring structures, but these are not the active form. Instead, open rings, in association with RPA, drive ssDNA annealing. Cryo-electron microscopy and biochemical analyses show that ssDNA binds to the positively charged channel of the open ring. The N-terminal domains of RAD52 drive annealing, while the C-terminal regions modulate the open-ring conformation and RPA interaction. RPA binds to RAD52 at the site of ring opening, with critical interactions between the RPA-interacting domain of RAD52 and the winged helix domain of RPA2. The structures of RAD52 closed and open rings were determined using cryo-EM. The open rings contain up to ten subunits, while the closed rings have eleven. The ssDNA-binding domain (DBD) of RAD52 is flexible and stabilizes ssDNA in the positively charged groove of the ring. The RAD52-ssDNA complex shows that ssDNA is stretched non-uniformly, with each quartet adopting the length of B-form DNA. Arg153 coordinates the phosphate backbone of the two central nucleotides, while Arg55 and Lys152 coordinate between quartet stacks. Mg²+ and surrounding water molecules are coordinated by residues from neighboring subunits. The oligomeric state of human RAD52 is influenced by protein concentration but not by ssDNA binding. The C-terminal region of RAD52 is structurally disordered but plays a critical role in RPA interaction and nuclear localization. Deletion mutants of RAD52 show that the RPA-interacting domain (RID) and C-terminal regions are essential for ring conformation and RPA interaction. The RID and C-terminal sequences are highly conserved in vertebrates. RAD52 open rings form more stable complexes with ssDNA and RPA compared to closed rings, explaining why RAD52-CRs do not cooperate with RPA in ssDNA annealing. The cryo-EM structure of the RPA-ssDNA complex reveals that RPA binds to the positively charged groove of RPA1 and RPA2. The RAD52-ssDNA-RPA complex shows that RPA binds at the site of ring opening, and