Billie Eilish participated in Calvin Klein's 'I Speak My Truth' #MyCalvins campaign in May 2019, where she shared her personal truth about wearing baggy clothes to maintain privacy and prevent public opinion on her body. Eilish stated, 'I never want the world to know everything about me... that's why I wear big baggy clothes. Nobody can have an opinion because they haven't seen what's underneath.'

Billie Eilish collaborated with Gucci, particularly with creative director Alessandro Michele, to create some of her most memorable outfits, including looks for the Grammys in March.

Billie Eilish collaborated with Uniqlo to create a UT collection in May 2020 that features six slouchy T-shirts for men, women, and children. The T-shirts showcase bright neon colors along with Billie Eilish's name, Blohsh merchandise prints, and Takashi Murakami's manga-style florals.