The challenge of controllable chemical synthesis of aluminum nanocrystals (Al NCs) has been met with only limited success. A major barrier is the absence of effective ligands to control the nucleation and growth of Al NCs. Here we demonstrate the size- and shape-controlled synthesis of monodisperse Al NCs using a polymer ligand, cumyl dithiobenzoate-terminated polystyrene (CDTB-PS). Density functional theory (DFT) calculations indicate that CDTB-PS shows selective absorption on Al{100} facets, inducing the formation of nanocubes and trigonal bipyramids. An excess of CDTB-PS, however, decreases the supersaturation of Al atoms, leading to the formation of OM facet-terminated octahedral and triangular plates. The concentration of the catalyst, titanium (IV) isopropoxide, determines the size of Al NCs by controlling the number of seeds. Depending on nanoparticle size, the solutions of Al NCs possess distinct colors, a characteristic feature of plasmonic nanomaterials. This robust and controlled chemical synthesis of Al NCs lays a foundation for Al as a sustainable plasmonic material for current and future applications.