The self‐assembly of block copolymers constitutes a timely research area in polymer science with implications for applications like sensing or drug‐delivery. Here, the unprecedented aggregation behavior of high molar mass block copolymer poly(N,N‐diethylacrylamide)‐b‐poly(4‐acryloylmorpholine) (PDEA‐b‐PAM) (Mn>400 kg mol−1) in organic solvent tetrahydrofuran (THF) is investigated. To elucidate the aggregation, dynamic light scattering, cryo‐transmission electron microscopy, and turbidimetry are employed. The aggregate formation is assigned to the unprecedented upper critical solution temperature behavior of PAM in THF at elevated concentrations (> 6 wt.%) and high molar masses. Various future directions for this new thermo‐responsive block copolymer are envisioned, for example, in the areas of photonics or templating of inorganic structures. [ABSTRACT FROM AUTHOR]