iHeartMedia announced this year's winners on iHeartRadio stations and social media platforms over Labor Day weekend.
Eilish, 18, won four awards, including Female Artist of the Year and Alternative Rock Album of the Year for When We Fall Asleep, Where Do We Go? She gave a short acceptance speech via remote video.
Swift, 30, took home Pop Album of the Year for Lover. She thanked fans for supporting Lover and her new album, Folklore, in her acceptance speech.
"That is so exciting and so generous of you, the fans, to vote and to care," the singer said.
"Folklore has been the way that I felt like I could still connect with people in this very disconnected time," she added, referencing the coronavirus (COVID-19) pandemic.
"This award is for you. You've been with through thick and thin, and we really can't put into words how much you mean to us," RM said.
Other winners included Lizzo, who took home Best New Pop Artist, Post Malone, who won Male Artist of the Year, the Jonas Brothers, who took home Best Duo/Group of the Year, and Luke Combs, who won Country Artist of the Year.
Here is the full list of 2020 iHeartRadio Music Awards winners:
Song of the Year: "Truth Hurts" by Lizzo
Female Artist of the Year: Billie Eilish
Male Artist of the Year: Post Malone
Best Duo/Group of the Year: Jonas Brothers
Pop Album of the Year: Lover by Taylor Swift
Best New Pop Artist: Lizzo
Alternative Rock Song of the Year: "Bad Guy" by Billie Eilish
Alternative Rock Album of the Year: When We All Fall Asleep, Where Do We Go? by Billie Eilish
Best New Rock/Alternative Rock Artist: Shaed
Rock Song of the Year: "Ghost" by Badflower
Rock Album of the Year: Fear Inoculum by Tool
Country Song of the Year: "Beautiful Crazy" by Luke Combs
Country Artist of the Year: Luke Combs
Country Album of the Year: What You See is What You Get by Luke Combs
Best New Country Artist: Morgan Wallen
Dance Song of the Year: "Close to Me" by Ellie Goulding, Diplo featuring Swae Lee
Dance Artist of the Year: Marshmello
Dance Album of the Year: World War Joy by The Chainsmokers
Hip-Hop Song of the Year: "Suge" by DaBaby
Hip-Hop Artist of the Year: Drake
Hip-Hop Album of the Year: Death Race for Love by Juice Wrld
Best New Hip-Hop Artist: DaBaby
R&B Song of the Year: "No Guidance" by Chris Brown featuring Drake
R&B Artist of the Year: H.E.R.
R&B Album of the Year: Free Spirit by Khalid
Best New R&B Artist: Summer Walker
Latin Pop/Reggaeton Song of the Year: "Con Calma" by Daddy Yankee and Katy Perry featuring Snow
Latin Pop/Reggaeton Artist of the Year: Ozuna
Best New Latin Pop/Reggaeton Artist: Rosalía
Regional Mexican Song of the Year: "A Través Del Vaso" by Banda Los Sebastianes
Regional Mexican Artist of the Year: Christian Nodal
Latin Album of the Year: X 100PRE by Bad Bunny
Best New Regional Mexican Artist: Banda Los Sebastianes
Label of the Year: Republic Records
Most Thumbed Up Artist of the Year: Post Malone
Most Thumbed Up Song of the Year: "Sunflower" by Post Malone and Swae Lee
Producer of the Year: Finneas
Songwriter of the Year: Louis Bell
Tour of the Year: Elton John
Best Lyrics: "10,000 Hours" by Dan + Shay and Justin Bieber
Best Cover Song: Sam Smith and Normani's cover of 5SOS' "Dancing with a Stranger"
Best Fan Army: BTSArmy for BTS
Best Music Video: "Boy with Luv" by BTS featuring Halsey
Social Star Award: Asher Angel
Best Remix: "Trampoline" by Shaed featuring Zayn
Favorite Tour Photographer: Zack Caspary (Why Don't We)
Favorite Music Video Choreography: "Kill This Love" (Blackpink) by Kyle Hanagami and Kiel Tutin