An electron donor/acceptor pair comprising perylene (Pe) and 9,10‐dicyanoanthracene (DCA) was specifically designed to construct organic charge‐transfer (CT) alloys via weak CT interaction through a solution co‐assembly route. By adjusting the molar ratio between Pe and DCA, we achieve color‐ and dimension‐tunable CT alloy assemblies involving one‐dimensional (1D) (DCA)1−x(Pe)x (0 ≤ x ≤10 %) microribbons and two‐dimensional (2D) (Pe)1−y(DCA)y (0 ≤ y ≤5 %) nanosheets as a consequence of energy transfer from DCA or α‐Pe to Pe‐DCA CT complex. Importantly, dimension‐related optical waveguiding performances are also revealed: continuously adjustable optical loss in 1D (DCA)1−x(Pe)x microribbons and successive conversion from isotropic waveguide to anisotropic waveguide in 2D (Pe)1−y(DCA)y nanosheets. The present work provides a desired platform for in‐depth investigation of light‐harvesting organic CT alloy assemblies, which show promising applications in miniaturized optoelectronic devices.