The synthesis of nonbenzenoid nanographenes (NGs) with a high density of nonhexagonal rings remains a significant challenge, leaving this structural class largely unexplored. In this work, we report the efficient synthesis of a novel nonbenzenoid isomer of decacyclene, namely cyclopenta [cd]azulene trimer (CPAT), derived from the natural product guaiazulene. The key step involves an acid–base buffer-assisted cyclotrimerization of cyclopenta[cd]azulen-2 (1H)-one that can be further extended to produce halogenated derivative, enabling structural tunability. The resulting molecules feature a unique backbone comprising nine nonhexagonal rings arranged around a central benzenoid core, representing the highest pentagon–heptagon ring density among all reported π-extended NGs with ten or more fused rings. Compared to the pristine decacyclene, the synthesized compounds exhibit excellent ambient stability, narrower bandgaps, and distinct aromaticity profiles. Moreover, femtosecond transient absorption measurements of CPAT-a reveal an ultrafast singlet-state relaxation (∼14 ps), significantly shorter than that of decacyclene, highlighting the pronounced impact of nonbenzenoid topology on excited-state dynamics. This study introduces a new family of nonbenzenoid NGs and paves the way for the synthesis of new sp² carbon allotrope featuring exclusively nonhexagonal ring systems.