It is our great pleasure to welcome you to CPSS 2021, the seventh ACM Cyber-Physical System Security Workshop, co-located with ACM AsiaCCS 2021. Due to the COVID-19 pandemic, the workshop will be held online, on 7 June 2021. The call for papers attracted 16 submissions with authors from 10 countries: China, Germany, India, Italy, Netherlands, Singapore, Sweden, Switzerland, United Kingdom, and the United States. Submissions were evaluated based on their significance, novelty, technical quality and relevance for the field. The review process was single-blinded. The program committee members have put in a significant effort in paper evaluation, and most of the papers received three reviews. Finally, we accepted six papers for presentation at the workshop, with an acceptance rate of 37.5%.