Guillermo Fesser Pérez de Petinto (Madrid; 1960) is a Spanish journalist known for utilizing humor while reporting news on Spanish radio throughout his innovative program called Gomaespuma. It was perceived as a breath of fresh air among the serious media of the time and it was an instant hit. Fesser studied journalism at the Universidad Complutense de Madrid. He received a Fulbright Scholarship to study film at the University of Southern California, Los Angeles.

With his partner Juan Luis Cano, Fesser created in 1984 Gomaespuma, a company that has produced shows and commercials for radio and television as well as books, music albums and cultural events. Both Fesser and Cano, lead since 2000 the Gomaespuma Foundation. This non profitable organization is mostly dedicated to facilitate an access to education to hundreds of children in Nicaragua and Sri Lanka. But Gomaespuma Foundation has supported another kind of programs like the drilling of water wells in Southern Senegal. To raise funds, Gomaespuma organizes cultural events as the Flamenco Festival Flamenco Pá Tós that promotes flamenco dance and music locally in Madrid (Spain), and internationally in cities like Beijing (China), Argel (Argelia) and Managua (Nicaragua). Profits from the festival help finance the educational projects.

He was the director, executive producer and one of the screenwriters of the film Cándida (2006). He is also brother of a film director Javier Fesser and the cultural promoter Alberto Fesser.

Guillermo Fesser is mostly known as a radio personality. For 25 years (1982–2007) he was hosting with his partner Juan Luis Cano one of the most popular radio shows in Spain: Gomaespuma. The show started as a weekly late night program in Antena 3 Radio in May 1982, and it got to its peak as a daily morning show on M-80 Radio, with over 1 Million listeners throughout the country. During the Gomaespuma years, Cano and Fesser traveled around the world to cover the news, interviewing key people like the King of Jordan and Vice President Al Gore just to name a few. Both, Fesser and Cano became personally involved in attempting to fight against poverty and injustice. This was what led them into creating the foundation that bears the name of their radio program Gomaespuma. Before, Fesser was the anchor with Juan Luis Cano, Santiago Alcanda and Jaime Barella of one of the first FM late night radio talk shows in Spain (in Francoist Spain the FM radio was not allowed to broadcast after midnight. In 1981, the law was changed). This radio show, El Flexo (the desk light), was perceived as a breath of fresh air among the up tied and serious media of the time and it was an instant hit for Radio Madrid. El Flexo is considered the ancestor of Gomaespuma. Fesser met his partner Juan Luis Cano in 1977 at Universidad Complutense de Madrid.

Guillermo Fesser directed in 2005 the feature film Cándida, a story based on his bestselling biography of a cleaning lady Cuando Dios Aprieta Ahoga Pero Bien (Memorias de Una Assistanta) (When God chokes, he chokes to death).

Previously he co-wrote with his brother Javier the screenplay of two big box office movies: El Milagro de P. Tinto [es] (1999) and La Gran Aventura de Mortadelo y Filemón (2003), both directed by his brother.

In 1994 Fesser produced, with the advice of Kermit Love, the puppeteer that created Big Bird for Sesame Street, a puppet show for Telecinco: Gomaespuma. Due to that experience, later he manipulated and gave his voice to the funny puppet that appeared in his brother first short film Aquel ritmillo [es]. He also worked as a script supervisor on El Secdleto de la Tlompeta [es], the multi-awarded second short film directed by Javier.

Fesser has translated and adapted several English and American films to his native language to be dubbed into Spanish. As Gomaespuma, with Cano, he has given his voice to many cartoon characters in children features as Chicken Run, Quest For Camelot, Cats & Dogs, and Chicken Run 2, and he has adapted and dubbed into Spanish some other films as the one from Sacha Baron Cohen Ali G Indahouse.