This book constitutes revised and selected papers from the 7th International Symposium on Security and Privacy in Social Networks and Big Data, SocialSec 2021, held in Fuzhou, China, in November 2021.The 16 full papers presented in this volume were carefully reviewed and selected from a total of 41 submissions. Such themes as privacy protection, security of AI, mobile social networks, Big Data system, applied cryptography, and others are covered in the volume.The papers are organized in the following topical sections: ​Applied Cryptography for Big Data; Big Data System Security; Forensics in Social Networks and Big Data; Privacy Protection in Social Networks; Security and Privacy in Big Database; Security of AI; Trust and Reputations in Social Networks.