We report a general strategy for spatiotemporal control of actin polymerization in vitro using photoactivatable actin. Caged actin was synthesized by chemically modified lysine residues on monomeric actin and released with focused ultraviolet (UV) illumination. Epifluorescence microscopy revealed nucleation and elongation of individual actin filaments (8 nm in diameter) after localized release of caged actin. We also used this strategy to generate branched filament structures by releasing caged actin in the presence of actin binding proteins. Controlled self-assembly of actin filaments represents a versatile "bottom-up" technique for constructing structural building blocks and functional templates for nanoscience applications.