The Workshop on Operator Theory, Complex Analysis, and Applications 2021 - WOTCA 2021 will be held at the Department of Mathematics of Instituto Superior Técnico - University of Lisbon on June 21-24, 2021. It aims to bring together researchers working in Operator Theory, Complex Analysis, and their applications, in particular Mathematical Physics, and to create an opportunity to highlight the current state of the art in these fields, to present open problems and to promote collaborative discussions. This meeting follows the line of six previous workshops, WOTCA 2010, WOTCA 2012, 2-Day OTCA 2013, WOTCA 2014, WOTCA 2016, WOTCA 2017 and WOTCA 2018. The main difference with respect to the previous WOTCA meetings will be that we will have online talks, which we expect to be the norm, but we plan to conjugate them with some in-person talks.