This volume contains papers of the 9th European Workshop on the Seismic Behaviour of Irregular and Complex Structures (9EWICS) held in Lisbon, Portugal, in 2020. This workshop, organized at Instituto Superior Técnico, University of Lisbon, continued the successful three-annual series of workshops started back in 1996. Its organization had the sponsorship of Working Group 8 (Seismic Behaviour of Irregular and Complex Structures) of the European Association of Earthquake Engineering.This international event provided a platform for discussion and exchange of ideas and unveiled new insights on...