The 32-year-old singer appeared to have lost weight in a little black-and-white belted dress and traded in her usual heavy make-up for a fresher and minimal look, reported the New York Daily News.
Aguilera attended the "Voice" event even though she will not be returning as a judge on the show for its fourth season.
She and Cee Lo Green have been replaced by Shakira and Usher.
Asked about her return on the show, she said, "I can't say anything right now. I can only do what feels right for me in the moment and right now what's important for me, and this is why I love the show, and why I'm coming back in support."