How cells balance the incorporation of actin into diverse structures is poorly understood. In budding yeast, a single actin monomer pool is used to build both actin cables involved in polarized growth and actin cortical patches involved in endocytosis. Here we report how Aim21/Tda2 is recruited to the cortical region of actin patches, where it negatively regulates actin assembly to elevate the available actin monomer pool. Aim21 has four polyproline regions and is recruited by two SH3-containing patch proteins, Bbc1 and Abp1. The C-terminal region, which is required for its function, binds Tda2. Cell biological and biochemical data reveal that Aim21/Tda2 is a negative regulator of barbed end filamentous actin (F-actin) assembly, and this activity is necessary for efficient endocytosis and plays a pivotal role in balancing the distribution of actin between cables and patches. Aim21/Tda2 also forms a complex with the F-actin barbed end capping protein Cap1/Cap2, revealing an interplay between regulators and showing the complexity of regulation of barbed end assembly.