This volume constitutes the proceedings of the 12th International Conference on Social Informatics, SocInfo 2020, held in Pisa, Italy, in October 2020. The 30 full and 3 short papers presented in these proceedings were carefully reviewed and selected from 99 submissions. The papers presented in this volume cover a broad range of topics, ranging from works that ground information-system design on social concepts, to papers that analyze complex social systems using computational methods, or explore socio-technical systems using social sciences methods.