"It gets a little dark," he told the crowd of "aw-ing" fans. When he wrote the song, he said, he was sitting on a tour bus on a rainy day after a set, thinking about a story of a couple in love for many years. Then, he wondered what it would feel like if the man's wife walked in and said "It's over."
Although Mendes clarified that nothing like that has happened to him (he's not married just yet), he launched into the ballad as girls swayed along.
Unbeknownst to his die-hard fans, the singer teased the new tune on Twitter back in June.
Watch Shawn Mendes Perform 'Stitches' Live: Billboard Exclusive
Soon after singing "Act Like You Love Me," Mendes surprised the crowd with another new song, "Memories." This is the first song he has ever performed without playing his guitar.
He said the new song was all about the memories that one creates with the person they love and keeping them forever. The up-and-coming pop star had issues with the sound before singing, but made jokes with the crowd while he fixed the volume. (Check out fan-shot videos of "Act Like You Love Me" and "Memories.")
It was a busy day for Mendes, as he came right to the venue from winning a Teen Choice Award for Choice Web Star. He also shared with the crowd that "Life of the Party" went platinum in the U.S. Mendes has had four Billboard Hot 100 debuts, and "Stitches" currently sits at No. 42 this week on the Hot 100 chart.
YouTube stars Jack & Jack were seen watching their friend's show from the audience and attracted many shouts from the young girls around them during Mendes' set.
Mendes is currently on his own tour, as well as joining Taylor Swift as an opening act on her 1989 World Tour. Check out more of his dates here.