This book constitutes the proceedings of the 9th International Symposium on Security and Privacy in Social Networks and Big Data, SocialSec 2023, which took place in Canterbury, UK, in August 2023. The 10 full papers and 4 short papers presented in this volume were carefully reviewed and selected from 29 submissions. They were organized in topical sections as follows: information abuse and political discourse; attacks; social structure and community; and security and privacy matters. Papers "Data Reconstruction Attack Against Principal Component Analysis" and "Edge local Differential Privacy for Dynamic Graphs" are published Open Access under the CC BY 4.0 License.