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A simple method was developed to produce polymeric nanoribbons and other nanostructures in water. This approach incorporates a perylene diimide (PDI) functionalized by triethylene glycol (TEG) as a hydrophobic supramolecular structure directing unit (SSDU) into the core of hydrophilic poly(N,N‐dimethylacrylamide) (PDMAc) chains using RAFT polymerization. All PDI‐functional polymers dissolved spontaneously in water, forming different nanostructures depending on the degree of polymerization (DPn): nanoribbons and nanocylinders for DPn=14 and 22, and spheres for DPn>50 as determined by cryo‐TEM and SAXS analyses. UV/Vis absorption spectroscopy was used to monitor the evolution of the PDI absorption signal upon dissolution. In solid form, all polymers show a H‐aggregate absorption signature, but upon dissolution in water, the shortest DPn forming nanoribbons evolved to show HJ‐aggregate absorption signals. Over time, the J‐aggregate band increased in intensity, while cryo‐TEM monitoring evidenced an increase in the nanoribbon's width. Heating the nanoribbons above 60 °C, triggered a morphological transition from nanoribbons to nanocylinders, due to the disappearance of J‐aggregates, while H‐aggregates were maintained. The study shows that the TEG‐PDI is a powerful SSDU to promote 2D or 1D self‐assembly of polymers depending on DPn through simple dissolution in water. [ABSTRACT FROM AUTHOR]