Improvisation is an extemporaneous and creative process for dealing with uncertainty. Confronted with complex and dynamic uncertainties, megaprojects are fertile soil for diverse improvisation involving multi-level stakeholders. To unveil the complex taxonomy of megaproject improvisation (MI), we first develop a three-dimension theoretical framework of MI drawing on classical improvisation theory, taking into account improvisational degree, organizational level, and project phase. Based on the framework, an abductive approach is adopted utilizing 20 expert interviews and 863 MI event collections to explore MI taxonomy and its evolutionary features over megaproject lifecycles. Specifically, 13 MI types are identified by integrating improvisational degree and organizational level dimensions, and the evolution of MI taxonomy is unveiled from the project lifecycle perspective, indicating that distinct megaproject phases appeal to corresponding MI types to respond to unique uncertain circumstances. This study provides a systematic understanding of MI for better dealing with uncertainties in megaprojects.