She is the youngest artist in history to write and record a soundtrack for a 007 film.
The US singer-songwriter, 18, was confirmed by Bond producers Michael G Wilson and Barbara Broccoli just hours after she teased her fans on social media that she was behind the song.
Eilish has written the song with her brother Finneas.
Wilson and Broccoli said: "We are excited to announce that Billie and Finneas have written an incredibly powerful and moving song for No Time To Die, which has been impeccably crafted to work within the emotional story of the film."
The Grammy-nominated music star said: "It feels crazy to be a part of this in every way. To be able to score the theme song to a film that is part of such a legendary series is a huge honour.
"James Bond is the coolest film franchise ever to exist. I'm still in shock."
Finneas added: "Writing the theme song for a Bond film is something we've been dreaming about doing our entire lives.
"There is no more iconic pairing of music and cinema than the likes of Goldfinger and Live And Let Die. We feel so so lucky to play a small role in such a legendary franchise, long live 007."
The film's director Cary Joji Fukunaga said: "There are a chosen few who record a Bond theme. I am a huge fan of Billie and Finneas.
"Their creative integrity and talent are second to none and I cannot wait for audiences to hear what they've brought - a fresh new perspective whose vocals will echo for generations to come."
The news was confirmed hours after Eilish teased her involvement in the franchise by posting a series of Bond actresses to her Instagram Stories.
She follows in the footsteps of artists including Adele and Sam Smith to record a Bond theme.