The song was released on Friday, May 10, and experts have accused the duo of ripping off a track of the name same by British pop star Cheryl Cole.
Fans took to Twitter to compare the two songs, pointing out the similarities.
"Wtf (what the f**k) happened to Ed Sheeran? Chorus is a rip off of I don't Care by Cheryl," tweeted one person, while another added: "When the chorus from the new @edsheeran and @justinbieber song starts, I just start singing the chorus from the @CherylOfficial song."
A third fan posted: "So... The new Ed Sheeran & Justin Bieber track is giving me MASSIVE Cheryl 'I Don't Care' vibes! #coincidence."
Another person advised former "Girls Aloud" star Cheryl to seek legal action, and tweeted: "@CherylOfficial girl you should sue ed sheeran he copied your song (sic)."
Cheryl's single was released in 2014, and made it to number one in the U.K. charts.
Meanwhile, this is the third time Justin and Ed have collaborated, having previously worked together on Justin's "Love Yourself" and Major Lazer's "Cold Water".
Fans' comments come just four months after a U.S. judge ruled a jury will decide whether or not Ed is guilty of ripping off Marvin Gaye's "Let's Get It On" for his smash hit single "Thinking Out Loud".
The British singer has also been accused of plagiarism over his songs "Photograph" and "Shape of You", and in January, Sheeran, Tim McGraw, and Faith Hill settled a copyright lawsuit over their song "The Rest of Our Life".
Sean Carey and Beau Golden accused the Brit of ripping off the melody for their 2014 song, "When I Found You", which they wrote for Jasmine Rae. Ed fired back with documents of his own, denying there was any similarity between the two songs.
McGraw and Hill were not defendants in the case, but all parties agreed to settle. The lawsuit has been dismissed.