Actin dynamics provide the driving force for many cellular processes including motility and endocytosis. Among the central cytoskeletal regulators are actin-depolymerizing factor (ADF)/cofilin, which depolymerizes actin filaments, and twinfilin, which sequesters actin monomers and caps filament barbed ends. Both interact with actin through an ADF homology (ADF-H) domain, which is also found in several other actin-binding proteins. However, in the absence of an atomic structure for the ADF-H domain in complex with actin, the mechanism by which these proteins interact with actin has remained unknown. Here, we present the crystal structure of twinfilin's C-terminal ADF-H domain in complex with an actin monomer. This domain binds between actin subdomains 1 and 3 through an interface that is conserved among ADF-H domain proteins. Based on this structure, we suggest a mechanism by which ADF/cofilin and twinfilin inhibit nucleotide exchange of actin monomers and present a model for how ADF/cofilin induces filament depolymerization by weakening intrafilament interactions.