Since the founding of ACM SIGMM in 1993, ACM Multimedia has been the worldwide premier conference and a key world event to display scientific achievements and innovative industrial products in the multimedia field. For the first time in its history, ACM Multimedia 2021 will be held in Chengdu, the capital city of the Sichuan Province in China. At ACM Multimedia 2021, we will present an extensive program consisting of technical sessions covering all aspects of the multimedia field via oral, video and poster presentations, tutorials, panels, exhibits, demonstrations, workshops, doctoral symposium, multimedia grand challenge, brave new ideas on shaping the research landscape, open source software competition, and also an interactive arts program stimulating artists and computer scientists to meet and discover together the frontiers of artistic communication. We will also add the industrial track to recognize those research works with significant industrial values.