In eukaryotic cells, RAD52 protein plays a central role in genetic recombination and DNA repair by (i) promoting the annealing of complementary single-stranded DNA and (ii) stimulation of the RAD51 recombinase. The single-strand annealing domain resides in the N-terminal region of the protein and is highly conserved, whereas the nonconserved RAD51-interaction domain is located in the C-terminal region. An N-terminal fragment of human RAD52 (residues 1-209) has been purified to homogeneity and, similar to the full-size protein (residues 1-418), shown to promote single-strand annealing in vitro. We have determined the crystal structure of this single-strand annealing domain at 2.7 A. The structure reveals an undecameric (11) subunit ring with extensive subunit contacts. A large, positively charged groove runs along the surface of the ring, readily suggesting a mechanism by which RAD52 presents the single strand for reannealing with complementary single-stranded DNA.