Because of their unique structural and optical properties, 1D perylene diimide (PDI) derivatives have gained attention for use in optoelectronic devices. However, PDI‐containing self‐assembled supramolecular systems often are of limited use because they have supramolecular architectures that are held together by weak noncovalent π–π stacking, hydrogen bonding, and hydrophobic interactions. As a result, they are intrinsically unstable under solution‐processing conditions. To overcome this limitation, a polydiacetylene (PDA)‐based strategy is developed to construct a solvent‐resistant and stable PDI assembly. For this purpose, first the monomer PDI–BisDA is generated, in which two polymerizable diacetylene (DA) units are covalently linked to a PDI core. Importantly, 254 nm UV irradiation of self‐assembled PDI–BisDA nanofibers forms solvent‐resistant and stable PDI–PDA fibers. Owing to the presence of PDA, the generated polymer fibers display an increased photocurrent. In addition, the existence of PDA and PDI moieties in the fiber leads to the occurrence of switchable on–off fluorescence resonance energy transfer (FRET) between the PDI and reversibly thermochromic PDA chromophores.