Semiconducting polymers have been widely investigated due to their intriguing optoelectronic properties and their high crystallinity that provides a strong driving force for self-assembly. Although there are various reports of successful self-assembly of nanostructures using semiconducting polymers, direct in situ self-assembly of these polymers into two-dimensional (2D) nanostructures has proven difficult, despite their importance for optoelectronics applications. Here, we report the synthesis of a simple conjugated homopolymer by living cyclopolymerization of a 1,6-heptadiyne (having a fluorene moiety) and its efficient in situ formation of large-area 2D fluorescent semiconducting nanostructures. Using high-resolution imaging tools such as atomic force microscopy and transmission electron microscopy, we observed the solvent-dependent self-assembly behaviors of this homopolymer; the identical starting polymer formed 2D nanosheets with different shapes, such as rectangle, raft, and leaf, when dissolved in different solvents. Furthermore, super-resolution optical microscopy enabled the real-time imaging of the fluorescent 2D nanosheets, revealing their stable and uniform shapes, fluorescence, and solution dynamics. Notably, we propose an orthorhombic crystalline packing model to explain the direct formation of 2D nanostructures based on various diffraction patterns, providing important insight for their shape modulation during the self-assembly.