Songwriter Teddy Geiger: 'I am transitioning'

"This is who I have been for a looooong time," Teddy Geiger wrote.

— -- Songwriter Teddy Geiger, who co-wrote all of Shawn Mendes' biggest hits, is transitioning.

Geiger, who has also written songs for One Direction and Maroon Five, shared an exchange on social media with a fan who asked, "Why do you look so different lately?"

"Because u asked nicely...I am transitioning," Geiger wrote in an Instagram post that was shared on Twitter. "I started talking about it with a couple of my close friends and family about a month ago and it's given me the courage to start the process. I feel like the next step is to tell all y'all. So here goes."

"Love it or hate it this is who I have been for a looooong time," Geiger added. "I love u guys. Talk sooooon byeeee."

Geiger has been a music industry presence since 2004, and has also done some acting. The artist has released three EPs and two albums, and in 2006, scored a top-30 hit with "For You I Will (Confidence)."

A rep for Geiger did not respond to ABC News' request for comment.