Teddy Geiger -- the singer/songwriter best known for the 2006 ballad "For You I Will" -- just let the world know ... he's started the process of transitioning to a woman.
Geiger made the announcement by responding to a fan's question on social media about why he looked different lately. He says he started talking with close friends and family about a month ago and it gave him the courage to forge ahead.
He added ... "Love it or hate it this is who I have been for a looooong time."
Teddy's remained busy in the music biz since his platinum hit, co-writing songs for One Direction and Shawn Mendes and soundtracks for "Fifty Shades Darker" and "Ghostbusters."
He also co-starred in the 2008 film "The Rocker" alongside Emma Stone.