The 17-year-old was eliminated from the ITV contest after falling into the bottom two of the public vote with Misha B.
Devlin performed a cover of 'Chasing Cars' by Snow Patrol in the sing-off, but judges gave their backing to Misha B's rendition of 'Out Here on My Own' from the Fame musical instead.
Louis Walsh was first to vote and explained to host Dermot O'Leary that he would "pick the person that is going to give the most to the competition and wants this the most", who he then identified as Misha B.
Tulisa Contostavlos also chose to save Misha B due to her having more "potential" outside of the show.
Next up was a visibly-emotional Kelly Rowland, who struggled to choose between her two acts.
"I know how great these two young ladies are," she started. "I've watched them grow over the weeks and I've seen how hard they work.
"Janet, I know how bad you want it and I admire you as a musician. [You're] showing what you're made of and the reason why you are here now... I absolutely love you and adore you and think you are so incredible as an artist."
After accepting that Misha B wanted to stay in The X Factor "so bad", Rowland was informed that she needed to make a decision soon or abstain from the vote, which would send Devlin home with the majority.
"I really can't make a decision," she finally said. "I love these girls, I don't know what to do. I can't [vote], I have to pass it, I'm sorry."
O'Leary confirmed that Rowland's refusal to vote, which received boos from the studio audience, had sealed Devlin's fate. Gary Barlow also confirmed that he would have sent Devlin home.
Speaking after the result, Devlin said that she was "obviously very confused" by how tonight's judges' vote had played out, adding: "I just can't wait to get out of the show and start getting my music out and start working on it. It's exciting."
Despite failing to save her, Rowland was still full of praise for the "amazing" and "beautiful" young singer.
"Janet Devlin's audition goes down in history as one of the most life-stopping, amazing performances," she said. "She froze everything in time when she had her first audition... Janet will be a recording star. She has a unique voice and such a beautiful gift."
Digital Spy users correctly predicted that Devlin and Misha B would finish in the bottom two in our exclusive poll.
The X Factor semi-final airs on Saturday, December 3 at 8pm on ITV1.