The 2021 IEEE Workshop on Nonlinear Circuit Networks (NCN'21) will be held on December 3-4, 2021, at Community Plaza, Shikoku University, Tokushima, Japan. The objective of this workshop is to provide a forum for exchange of the latest results related to nonlinear circuit networks. Papers describing original works in all aspects of nonlinear circuit networks are invited. The NCN has been founded as a closed internal workshop by Prof. Ushida and Prof. Tanaka in 1995. In 2006, NCN'06 was organized as the first open NCN workshop sponsored by the IEEE CAS Society Shikoku Chapter preceded by 17 closed NCN workshops held in Tokushima, Tokyo, Nobeyama and Karuizawa. During 2007-2020, a closed NCN workshop was held in Tokyo. After the successful NCN'06, fifteen open NCN workshops (NCN'07 - NCN'20) were organized annually by the IEEE CAS Society Shikoku Chapter in Tokushima. The NCN'21 is the sixteenth open NCN workshop.