Whoa. Not sweet!

Adam Levine was signing autographs for fans outside Jimmy Kimmel when Pow! Right in the kisser -- with powdered sugar.

There were some scary moments for the "Sugar" singer. After all, nowadays, you never know what's in a white powdery substance.

Reports TMZ:  Unclear if the attacker was protesting on behalf of any particular group -- or just some idiot prankster -- but he was quickly detained by security until police arrived and arrested him for battery. Adam continued into the studio for his scheduled performance.