Ms Cowell, who studied MA Narrative Illustration at the university, is best known for her How to Train Your Dragon books, which have been adapted into film and television series. Her work has been published in 38 languages.
Ms Cowell follows Brighton alumnus and honorary graduate Chris Riddell, who held the Children’s Laureate role from 2015 to 2017.
After being unveiled at a ceremony at Shakespeare’s Globe in London, Ms Cowell told BBC News: "Books and reading are magic and this magic must be available to absolutely everyone.
"I'm honoured to be chosen... I will be a laureate who fights for books and children's interests with passion, conviction and action. Practical magic, empathy and creative intelligence, is the plan."
Ms Cowell also stressed her desire to review the role of primary school libraries and to ensure that all children have access to literature regardless of “socio-economic background”.
The university’s Vice-Chancellor Professor Debra Humphris said: “We are very proud of our alumna Cressida Cowell for being named Children’s Laureate. Along with her fellow Brighton graduate Chris Riddell’s recent stint as Laureate, Cressida’s honour is testament to the practical wisdom taught on our arts degrees.