Cyanallagma demoiselle sp. nov. (holotype male deposited in DZUP: Brazil, São Paulo State, Cananéia, Ilha do Cardoso State Park), a new small greenish blue and black damselfly, is described, illustrated, and diagnosed based on males and females from the southeastern Atlantic Forest. This new coenagrionid is named after one of the most celebrated projects, the Demoiselle 20 or libellule aircraft, designed by the Brazilian inventor and aviation pioneer Alberto Santos-Dumont (1873–1932). This is the third new odonate species discovered in the same restinga-like formation at São Paulo, and like many other odonates from this assemblage, C. demoiselle sp. nov., appears to be a typical inhabitant of this type of environment. Due to its sharing many characteristics with other Cyanallagma, the new species can be considered a chimera. Its body coloration and genital ligula are similar to those of C. trimaculatum, whereas its caudal appendages closely resemble those of C. nigrinuchale. Despite of recent advances in taxonomic knowledge about Cyanallagma, this study highlights the need for better understanding the morphological correspondences or homologies among the structures of caudal appendages.